Interferometer  Study 


OF 

Radiations  in  a Magnetic  Field 


JOHN  CUTLER  SHEDD 


A Thesis  Submitted  for  the  Degree  of  Doctor  of  Philosophy 
University  of  Wisconsin 
1899 


[Reprinted  from  the  Physical  Review,  Vol.  IX.,  Nos.  1 and  2,  1899.] 


Press  of 

The  New  Era  Printing  Company 
Lancaster,  Pa. 


<avas^ 


PREFATORY  NOTE. 

The  historic  interest  in  the  class  of  experiments  to  which  the  fol- 
lowing research  belongs,  consists  in  the  importance  of  establishing 
the  electromagnetic  theory  of  light  upon  an  experimental  basis. 

The  first  link  in  this  connecting  chain  is  furnished  by  Faraday’s 
discovery  of  the  magnetic  rotation  of  the  plane  of  polarization.  A 
second  link  is  furnished  by  the  experimentally  determined  ratio  of 
electromagnetic  to  electrostatic  units  as  expressed  by  the  equation 

V = i / ■ s/  Km 

where  V — velocity  of  light  in  a non-conducting  medium; 

K—  dialectric  inductive  constant  of  the  medium  ; 
m = magnetic  inductive  constant  of  the  medium. 

A third  link  is  furnished  in  Hertz’  epoch-making  experiments  on 
electrical  waves.  The  influence  ©f  a magnetic  field  of  force  upon  a 
source  of  light  placed  within  it,  is  a fourth  link,  and  marks  one  of 
the  most  important  advances  that  has  been  made  since  the  electro- 
magnetic theory  was  first  set  forth. 

A further  importance  is  to  be  attached  to  such  experiments  in 
that  much  light  will  be  thrown  upon  the  motions  of  the  ultimate 
particles  of  matter,  and  possibly  upon  the  ultimate  structure  of 
matter  itself. 


TABLE  OF  CONTENTS. 

Page 

Prefatory  Note. 

Part  One.  Historical  Survey. 

History,... i 

Lorentz’s  Theory, 3 

Methods, 5 

Experimental  Results, 7 

Modified  Theories, 10 

Summary, 14 

Part  Two.  Experimental  Work. 

Introductory  Note, 15 

Outline  of  Work,  15 

Section  I.  Preliminary  Survey,  16 

Summary, 19 

Section  It.  The  Interferometer  Method, 20 

Adjusting  the  Interferometer, 22 

Visibility  Curves,  23 

Polarization,  26 

Summary, 27 

Measurement  of  Change  of  Wave-length, 28 

Section  III.  Comparison  of  Magnetic  Shift  at  Different  Tem- 
peratures,  32 

Summary, 37 

Section  IV.  Measurements  of  Magnetic  Shift, 38 

Ratio  of  Ionic  Charge  to  Ionic  Mass, 43 

Polarization, 43 

Summary, 45 

Recapitulation,  45 

APPENDIX. 

I.  Extract  from  Life  of  Faraday, 47 

II.  Extract  from  M.  Ch.  Fievez, 47 

III.  Extract  from  Perot  and  Fabry, 48 

BIBLIOGRAPHY. 

I.  The  Interferometer, 50 

II.  Radiations  in  a Magnetic  Field, 50 


AN  INTERFEROMETER  STUDY  OF  RADIATIONS  IN 
A MAGNETIC  FIELD.  I. 

John  C Shedd. 

Part  One.  Historical  Survey. 

IN  1845  Faraday  discovered  the  rotation  of  the  plane  of  polar. 

ization  due  to  a magnetic  field  of  force.  This,  perhaps,  sug- 
gested the  further  experiment  as  to  the  effect  of  a magnetic  field 
upon  a source  of  radiation  placed  within  it.1 

In  the  light  of  what  is  now  known  Faraday’s  repeated  failure  to 
obtain  positive  results  from  this  last  experiment  may  be  ascribed  to 
the  low  dispersive  power  possessed  by  his  apparatus,  and  to  the  com- 
paratively low  temperature  of  the  gas  flame  used  as  a source  of 
radiation. 

In  1865  Maxwell  propounded  the  electromagnetic  theory  of 
light,  which  not  only  correlated  all  hitherto  observed  phenomena, 
but  also  furnished  a scientific  basis  for  future  research. 

In  1875  Professor  Tait  2 presented  a paper  “ On  a Possible  In- 
fluence of  Magnetism  on  the  Absorption  of  Light”  which,  while 
not  realized  by  him  experimentally  is  of  interest  in  this  connection. 
He  says  in  part : 

“ The  explanation  of  Faraday’s  rotation  of  the  plane  of  polariza- 
tion of  light  by  a transparent  diamagnetic  requires,  as  shown  by 

1 Appendix  I. 

2 Proc.  Roy.  Sec.  Edinburgh.  Sessions  1875-76,  p.  16S. 


2 


JOHN  C.  SHEDD . 


[Vol.  IX. 


Thomson,1  molecular  rotation  of  the  luminiferous  medium.  The 
plane  polarized  ray  is  broken  up,  while  in  the  medium,  into  its 
circularly  polarized  components,  one  of  which  rotates  with  the 
ether  so  as  to  have  its  period  accelerated,  the  other  against  it  in  a 
retarded  period.  Now,  suppose  the  medium  to  absorb  one  definite 
wave-length  only,  then — if  the  absorption  is  not  interfered  with  by 
the  magnetic  action — the  portion  absorbed  in  one  ray  will  be  of  a 
shorter,  in  the  other,  of  a longer  period  than  if  there  had  been  no 
magnetic  force  ; and  thus,  what  was  originally  a single  dark  absorp- 
tion line  might  become  a double  line,  the  components  being  less 
dark  than  the  single  one.” 

The  line  of  reasoning  here  presented,  if  applied  to  a source  of 
radiation  (instead  of  absorption)  placed  in  a magnetic  field  would 
give  an  analogous  solution  of  two  bright  lines  of  less  intensity  than 
the  original  line. 

The  first  experimental  results  in  this  field  of  work  were  ob- 
tained by  M.  Fievez2  in  188^—86.  His  observations  consisted  in  a 
broadening  of  the  spectral  line,  and  increased  brilliancy  of  illumina- 
tion. No  observations  are  recorded  as  to  the  state  of  polarization. 

The  phenomena  of  reversal  and  increased  brightness  of  the  lines, 
while  undoubtedly  precipitated  by  the  action  of  the  magnetic  field, 
were  probably  due,  primarily,  to  changes  of  temperature  and  den- 
sity. The  broadening  of  the  line,  however,  was  undoubtedly  a 
genuine  magnetic  effect,  and  as  Preston 3 remarks,  had  Fievez  been 
acquainted  with  the  theory  of  the  subject  the  whole  question  would 
have  been  settled  in  1885. 

Fievez  does  not  seem  to  have  been  familiar  with  what  Faraday 
had  done  as  regards  the  state  of  polarization,  nor  to  have  taken  any 
special  precautions  against  spontaneous  reversals  : yet,  while  his 
work  may,  in  the  light  of  recent  investigations,  appear  meager,  it 
deserves  an  important  place  in  the  history  of  the  subject,  and  must 
sooner  or  later  have  led  to  the  very  results  reached  by  Zeeman. 

A paper  entitled  “ Causes  of  Double  Lines  and  Close  Satellites 

1 Reprint  Thomson’s  Papers  on  Electrostatics  and  Magnetism.  Second  Edition,  p. 
423,  Footnote. 

2 Appendix  II. 

’Phil.  Mag.  (5),  45,  1898,  P-  338- 


No.  i.] 


INTERFER  OME  TER  S TUD  V 


3 


in  the  Spectra  of  Gases,”  by  G.  J.  Stoney,1  though  not  considering 
the  special  case  of  magnetic  forces,  has,  nevertheless,  a theoretical 
bearing  upon  the  subject.. 

Of  still  greater  importance  are  the  writings  of  Lorentz,2  pub- 
lished in  1892  and  1895,  inasmuch  as  they  were  a guide  to  Zeeman 
in  his  experiments. 

In  March,  1897,  Dr.  P.  Zeeman3  communicated  to  the  Philosoph- 
ical Magazine  the  results  of  a research  that  has  proved  wonderfully 
fruitful  in  his  own  hands  and  also  in  the  hands  of  others.  Zeeman 
was  familiar  with  Faraday’s  experiments,  but  did  not*  know  of 
Fievez’s  work.  He  was  influenced,  as  a matter  of  course,  by  Max- 
well’s electromagnetic  theory  of  light,  and  in  particular  was  guided 
by  Lorentz’ s exposition  of  it.  His  first  experiments  were  identical 
with  those  of  Faraday,  excepting  that  he  had  a Rowland  grating 
of  14,000  lines  to  the  inch,  and  hence  had  a much  greater  disper- 
sion. With  this  arrangement  of  the  apparatus,  a broadening  of  the 
spectral  line  was  observed,  similar  to  that  seen  by  Fievez.  Guided, 
however,  by  Lorentz’s  theory,  Zeeman  tested  the  state  of  polari- 
zation of  the  broadened  line,  and  found  that,  when  viewed  parallel 
to  the  lines  of  force,  the  edges  of  the  line  were  circularly  polarized 
as  predicted  by  Lorentz’s  theory. 

Lorentz' s Theory. — In  this  theory  it  is  assumed  that  light  vibra- 
tions are  the  vibrations  of  electrically  charged  ions  of  definite  mass. 
Thus  suppose  such  an  ion  having  a charge  e and  mass  m,  to  vibrate 
with  simple  harmonic  motion  about  its  center  of  equilibrium.  Such 
an  ion  moving  in  a magnetic  field  would  then  experience  mechan- 
ical forces,  which  would  cause  a change  of  period  of  vibration.  The 
amount  of  this  change  of  period  would  depend  upon  the  ratio  ejm, 
and  the  measurement  of  the  change  of  period  would  give  a knowl- 
edge of  this  ratio. 

The  equations  giving  the  modified  period  are  derived  as  follows  ,4 

In  Fig.  1 let  the  origin  be  at  o and  the  axis  of  Z be  parallel  to 

1 Sci.  Trans.  Roy.  Dublin  Soc.,  Vol.  IV.,  p.  563,  1891. 

2 Lorentz,  “ La  Theorie  electromagnetique  de  Maxwell.”  Leyden,  1892.  “ Versuch 

einer  Theorie  der  electrichen  und  optischen  Erscherungen  in  bewegten  Korpern.”  Ley- 
den, 1895. 

3 Phil.  Mag.  (5),  43,  p.  226.  Same  art.  Astro.  Phys.  Jr.,  5,  p.  332,  1897. 

4 See  Zeeman,  Phil.  Mag.  (5),  43,  p.  226. 


4 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Fig.  1. 


the  lines  of  force,  the  axes  of  X and  Y being  perpendicular  to  this 
direction.  Then  if  H be  the  strength  of  field,  the  equations  of  mo- 
tion relative  to  the  X and  Y axes  are 


in 

in 


d2x 

d2y 

~d? 


dx 


= -K2v-  eH 


dt 


(0 


The  term  K is  the  coefficient  of  elasticity  of  the  ion,  the  second 
term  gives  the  mechanical  force  due  to  the  magnetic  field. 

The  solution  of  these  equations  yields  for  the  period  of  vibration 
the  following  values  : 

If  H=  o,  T = — —r  (2) 


If  H be  not  zero,  and  we  regard  all  forces  in  the  X Y plane  as 
symmetrcial  with  respect  to  the  axis  of  Z,  then 


T 


in «/  in 


K 


eH  \ 
2 K %/  in  ' 


(3) 


The  meaning  of  equations  (2)  and  (3)  is  as  follows  : 

If  we  analyze  along  the  three  coordinate  axes,  the  motion  of 
the  moving  ion,  which  by  virtue  of  this  vibration  is  emitting  light, 
we  shall  have  three  component  vibrations  of  equal  period.  We 
may  next  compound  the  two  components  which  lie  in  the  X Y 
plane  into  circular  motion,  and  since  we  do  not  know  the  sense  of 
rotation,  we  may  conceive  it  to  be  in  both  directions  simultaneously.1 

If  the  source  of  light  be  viewed  in  a direction  parallel  to  the 
magnetic  field,  the  component  whose  motion  is  parallel  to  the  axes 
of  ^ is  not  effective,  and  the  ray  is,  therefore,  composed  of  circularly 

1 The  sense  of  rotation  will  depend  upon  the  sign  of  the  charge  e upon  the  iron. 


No.  i.] 


INTERFEROMETER  STUDY. 


5 


polarized  components,  whose  periods  of  rotation  are  the  same,  and 
for  zero  magnetic  field  have  the  value  given  by  equation  (2).  If 
the  magnetic  field  be  equal  to  Ht  then  the  periods  become  changed, 
and  are  given  by  equation  (3).  The  periods  being  changed  by 
equal  and  opposite  amounts,  the  original  spectral  line  becomes  two 
lines  of  equal  intensity,  symmetrically  situated  with  respect  to  the 
original  unmagnetized  line  ; the  components  being  circularly  polar- 
ized in  opposite  senses. 

Viewed  in  a direction  normal  to  the  magnetic  field,  the  component 
parallel  to  the  axis  of  Z,  and,  therefore,  parallel  to  the  axis  of  the 
magnetic  whirl,  is  not  affected  by  it,  is  unaltered  in  period  and  ap- 
pears as  a plane  polarized  ray.  The  two  circular  components,  being 
now  viewed  parallel  to  the  X Y plane,  also  appear  as  plane  polar- 
ized rays,  the  plane  of  polarization  being  normal  to  that  of  the  Z 
component. 

Thus  viewed,  the  spectral  line  is  broken  up  into  three  component 
lines,  and  if  the  magnetic  field  is  of  sufficient  intensity  there  will  be 
seen  three  distinct  lines,  the  two  outlying  ones  having  their  vibra- 
tions in  a plane  normal  to  the  magnetic  field,  and  the  central  one 
in  a plane  parallel  to  it.  If  the  magnetic  field  is  not  powerful 
enough  to  separate  the  three  components  they  will  overlap,  and  the 
line  appears  merely  broadened.  The  components  may,  however, 
be  isolated  by  means  of  a nicol  prism. 

In  his  earlier  work  1 Zeeman  obtained  only  a broadened  line,  but 
by  means  of  the  nicol  he  was  able  to  test  the  state  of  polariza- 
tion ; and,  later,2  he  succeeded  in  obtaining  both  the  doublet  par- 
allel to  the  magnetic  field,  and  the  triplet  normal  to  the  field,  as 
predicted  by  Lorentz’s  theory. 

Methods. — In  the  further  progress  of  the  work  two  radically  dif- 
ferent methods  have  been  developed. 

The  first  is  that  followed  by  Zeeman  and  many  other  investiga- 
tors. It  consists  in  photographing  the  spectral  lines,  and  in  meas- 
uring the  separation  of  the  “ magnetized”  components  by  means  of 
the  micrometer  dividing  engine.  The  chief  merit  of  this  method  is 
that  a permanent  record  is  secured  in  the  photograph.  It  is  also 

1 Phil.  Mag.  (5),  43,  p.  226. 

2 Phil.  Mag.  (5),  44,  pp.  55  and  255  ; 45,  p.  197. 


6 


JOHN  C.  SHEDD. 


[Vol.  IX. 


an  advantage  that  the  phenomenon  is  observed  directly.  The  limi- 
tations of  the  method  are  : First , the  fact  that  the  quantity  to  be 
measured  is  minute,  and  the  micrometer  method  of  direct  measure- 
ment is  of  necessity  limited  in  range  ; Second , it  frequently  happens 
that  the  components,  whose  distances  apart  are  to  be  measured,  are 
so  nebulous  as  to  make  it  exceedingly  difficult  to  make  exact  mi- 
crometer settings.  Third , the  time  of  exposure  necessary  is  some- 
times so  long  as  to  make  the  method  prohibitory. 

The  second  method  is  due  to  Professor  A.  A.  Michelson,  and 
may  be  called  the  Interferometer  Method.  It  has  been  used  suc- 
cessfully with  magnetic  fields  far  too  weak  to  give  any  sensible  effect 
by  the  direct  method,  and  has  been  shown  to  have  a delicacy  and 
sensitiveness  far  in  excess  of  any  photograph.  The  method,  as 
used  by  Professor  Michelson,  consists  in  obtaining  the  visibility 
curves  of  the  spectral  line,  both  when  unmagnetized,  and  also  with 
fields  of  different  intensity.  These  visibility  curves  are  then  ana- 
lyzed,1 and  the  distribution  of  light  at  the  source  of  illumination  ob- 
tained. This  gives  directly  the  various  “ magnetic”  components 
of  the  line  : By  means  of  a nicol  prism  the  two  planes  of  polariza- 
tion may  be  separately  examined. 

The  advantages  of  this  method  are — briefly — First , the  visibility 
curve  enables  the  separation  of  lines  not  hitherto  resolved  by  any 
other  method.  Second , the  eye  is  the  instrument  of  investigation, 
and  hence  there  is  no  need  of  long  exposure  as  in  the  case  of  taking 
photographs.  Third , any  change  of  polarization — or  other  effect — 
taking  place  during  the  period  of  observation  may  be  detected,  while 
the  photographic  process  is  necessarily  an  integrating  method. 

The  disadvantages  are  : First , the  method  is  an  indirect  one,  i.  e., 
the  observations  are  made  not  on  the  lines  themselves  but  on  in- 
terference fringes  produced  by  them.  Second , the  accurate  estimate 
of  a visibility  curve  is  by  no  means  an  easy  matter,  and  the  rare 
success  attained  by  Professor  Michelson  has  been  equalled  by 
no  one  else,  and  can  only  be  approached  by  practice  and  great, 
patience.  Third , the  record  of  the  instrument  is  not  automatic,  as 
in  the  photograph,  and  is  subject  to  the  personal  error  of  the  ob- 
server. Fourth , the  reflection  from  the  half  silvered  surface  of  the  in- 

1 Ph.il.  Mag.  (5),  34,  p.  280,  1892.  Astro.  Phys.  Jr.,  7,  p.  129,  1898. 


No.  i.] 


IJVTEKFER  OME  TER  S TUD  V 


7 


terferometer  affects  the  two  beams  polarized  in  perpendicular  planes , 
to  a different  degree,  so  that  when  both  beams  are  simultaneously 
observed  they  have  not  their  normal  ratio  of  brightness,  with  the  re- 
sult that  the  fringes  are  correspondingly  deceptive. 

In  the  hands  of  an  experienced  observer  the  interferometer  is  un- 
doubtedly the  most  powerful  instrument  of  attack  that  is  available 
at  present,  unless,  indeed,  Professor  Michelson  has  presented  in  the 
Eschelon  Plate  Spectroscope,  an  instrument  of  equally  great  value. 
Under  circumstances  less  favored  than  those  enjoyed  by  Profes- 
sor Michelson,  it  is  difficult  to  see  how  the  Interferometer  Method 
as  he  uses  it,  can  be  successfully  used.  There  are,  however,  modi- 
fications rendering  this  instrument  more  available,  which  have  been 
used  by  the  present  writer. 

Experimental  Results. — The  agreement  between  theory  and  ex- 
periment presented  by  Zeeman’s  early  experiments  was  truly  re- 
markable, and  the  apparent  simplicity  of  the  phenomenon  seemed 
equally  worthy  of  notice.  This  apparent  simplicity,  however,  was 
soon  found  not  to  be  true  of  all  spectral  lines,  and  more  complex 
forms  were  found.  Exceptions  were  also  found  to  the  state  of 
polarization  as  first  described  by  Zeeman. 

The  first  observer  to  note  a departure  from  the  normal  form  was 
M.  Cornu.1  His  apparatus  was  similar  to  that  of  Zeeman  except- 
ing that  he  used  a double  image  prism,  and  was  thus  able  to  ob- 
serve both  planes  of  polarization  simultaneously.  For  a magnetic 
field  strength  of  1 3,000  C.  G.  S.  units  he  observed  that  the  sodium 
line,  Dl  when  viewed  normal  to  the  magnetic  field  was  a quadruple, 
the  inner  components  being  polarized  perpendicular  to  the  mag- 
netic field  and  the  outer  ones  parallel  to  this  direction.  The  line, 
D2  he  found  to  be  a hazy  triple  with  each  member  perhaps  doubled. 

M.  Cornu  was  soon  followed  by  others.  Preston2  succeeded  in 
photographing  as  many  as  five  different  types,  and  Michelson 3 
with  the  interferometer  showed  three  well  marked  groups  of  lines. 

As  regards  polarization  Becquerel  and  Deslandres 4 have  found 

1 Astro.  Phys.  Jr.,  6,  p.  378,  1897;  7,  p.  163,  1898. 

2 Phil.  Mag.  (5),  45,  p.  330,  1898;  47,  p.  165,  1899. 

3 Phil.  Mag.  (5),  44,  p.  109,  1897,  same  Art.  Astro.  Phys  Jr.,  6,  p.  48,  1897  ; Phil. 
Mag.  (5),  45,  p.  348,  1898,  same  Art.  Astro.  Phys.  Jr.,  7,  p.  131,  1898. 

4 C.  R.,  April  4,  1898. 


8 


JOHN  C.  SHEDD. 


[VOL.  IX. 


that  one  of  the  iron  lines  when  viewed  perpendicularly  to  the  mag- 
netic field  becomes  a triple,  in  which  the  usual  state  of  polarization 
is  reversed.  The  same  phenomenon  has  also  been  observed  at  the 
Johns  Hopkins  1 University. 

The  classification  of  lines  as  given  by  Preston  is  shown  in  Fig. 
2,  who  thus  describes  them. 


Fig.  2. 


In  i we  have  the  normal  triplet.  In  2 we  have  the  weak  middled  ‘quartet’  in 
■which  nearly  all  the  light  is  concentrated  in  the  two  side  lines.  Next  we  have  in  3 the 
doublet  in  which  the  central  line  has  completely  disappeared.  Next  in  4 we  have  the 
double  doublet,  or  two  pairs  of  fine  lines,  and  finally  in  5 the  sextet  or  three  pairs  of 
equally  spaced  sharp  lines.” 


Preston  2 cites  the  following  as  examples  of  the  above  types 

Type  1,  Cd,  4678  ; Mg,  5167  ; Zn,  4680  and  the  vast  majority 
of  other  lines.  Type  2,  Mg,  5183  ; Cd,  5086;  Zn,  4810.  Type 
4,  Mg,  5173  ; Cd,  4800;  Zn,  4722.  In  his  latest  work3  Preston 
seems  to  restrict  his  classification  to  three  types,  viz. : “Diffuse 
triplets,”  “quartets”  and  “pure  triplets.”  It  may  be  that  types  3 
and  4 are  modified  forms  of  the  same  type,  as  also  2 and  5.  This 
would  leave  but  three  types. 

The  types  found  by  Michelson  4 are  shown  in  Fig.  3.  The  up- 
per curves  are  taken  by  the  interferometer  and  the  lower  by  the 
Echelon  plate  spectroscope.  The  spectral  lines  are  viewed  normal 
to  the  magnetic  field  in  both  figures  2 and  3. 

1 Astro.  Phys.  Jr.,  8,  p.  48,  1898. 

2 Phil.  Mag.  (5),  45,  p.  330. 

P Phil.  Mag.  (5),  47,  p.  178.  See  also  Nat.,  Vol.  59,  p.  226,  where  seven  types  are 
given. 

4 Astro.  Phys.  Jr.,  7,  p.  136,  1898.  Nat.,  March  9,  1899. 


No  i.] 


INTERF'EROME  TER  S TED  V. 


9 


L 


i 


JL- 

1 

^aI\a/\aA/\/\  - 

B 

.4  .3.2  .1  B.l.2.3.4 

B ' 

TYPE 

1 

TYPE  II 

type  nr 

PLANES  OF  POLARIZATION-!  a 
( B 

_J A 

= EQUITORIAL 
= AXIAL 

A 

B 

qrEEN  _ 

JIM 

A 

A « 

blue  — 

A 

All 

B 

A 

B 

II 

_JL 

A/V  . 

B 

Fig.  3. 

X 

The  following  are 

: some 

examples  : 

Type  I. 

Type  II. 

Type  III. 

Hg 

yellow  line. 

Hg 

violet  line. 

Hg  green  line. 

Cd 

red  line. 

Cd 

blue  line. 

Cd  green  line. 

Zn 

red  line. 

Zn 

blue  line. 

Mg  green  line  (5 183) 

Au 

green  line. 

Na 

yellow  line. 

Ag 

yellow  line. 

Au 

yellow  line. 

Ag 

green  line. 

Michelson  adds  a fourth  type  in  which  a broad  or  complex  line  is 
simplified  or  narrowed  in  the  magnetic  field.  Examples  of  this  are 
Cu  yellow  line  and  Mn  green  line  (5340).  This  effect  is  true  of 
the  central  member  of  the  triplet  in  the  case  of  these  two  lines. 

If  now  a comparison  be  made  between  such  lines  as  have  been 
observed  by  several  persons  the  following  rather  meager  data  are 
found. 


IO 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Table  I. 


Lines. 

Type. 

Field  Strength. 

Observer. 

Zn.  4810. 

I Diffuse  triplets. 

Preston. 

6 6 6 6 

Type  II 

10,000. 

Mich  el  son. 

Cd.  5086. 

Diffuse  triplets. 

Preston 

“ “ 

Type  III. 

10,000. 

Michelson. 

“ 4800. 

Quartets. 

Preston. 

66  66 

Type  II. 

10,000. 

Michelson. 

Na.  Dj^ 

Type  II. 

10,000. 

Michelson. 

“ “ 

Quartet. 

13,000. 

Cornu. 

“ D2 

Type  II. 

10,000. 

Michelson. 

6 6 66 

Sextet. 

13,000. 

Cornu. 

The  results  of  such  a comparison  are  of  little  value,  unless  the 
conditions  as  regards  the  source  of  radiation  and  the  magnetic  field 
strength  are  known  to  be  the  same.  Two  things  are,  however,  ap- 
parent. 

I.  The  phenomenon  is  by  no  means  as  simple  as  was  at  first  sup- 
posed by  Zeeman. 

II.  The  superiority  of  the  Interferometer  Method  as  regards  re- 
solving power  is  shown. 

Modified  Theories. — Having  found  that  the  extremely  simple  de- 
ductions from  Lorentz’s  theory,  as  made  by  Zeeman,  do  not  com- 
prehend the  observed  phenomena,  it  becomes  necessary,  either  to 
present  a new  hypothesis  or  suitably  to  modify  the  old  one.  This 
modification  has  been  made  by  Lorentz,* 1 II.  Larmor2  and  others,  while 
Preston3  has  pointed  out  that  the  paper  of  Dr.  Stoney,4  already 
cited,  anticipates  the  desired  theory. 

Dr.  Stoney  considers  the  effect  of  perturbing  forces  upon  an  ion,, 
moving  in  an  elliptical  orbit  under  the  action  of  a central  force, 
which  is  proportional  to  the  distance.  If  the  perturbing  force  be 
such  as  to  cause  the  orbit  to  rotate  in  its  own  plane,  then  the  spec- 
tral line  becomes  a doublet.  Thus  if  the  ion  be  moving  with  an. 

1 Proc.  Roy.  Acad.  Sci.  Amsterdam,  June  25,  1898.  Also  Astro.  Phys.  Jr.,  9,  p.  37. 
Wied.  Ann.,  Bd.  LXIII.,  p.  278,  1897. 

2 Phil.  Mag.  (5),  44,  p.  503. 

3 Phil.  Mag.  (5),  47,  p.  171. 

4Trans.  Roy.  Soc.  Dublin,  Vol.  IV.,  p.  563.  See  also  Preston,  Phil.  Mag.  (5),  47r- 
p.  171. 


No.  i.] 


INTER FER  OR/E  TER  S TUB  V 


I I 


angular  velocity  J2  then  U = 2 t:N  where  N is  the  frequency  of  the 
rotation.  If  now  an  angular  velocity  of  to  be  impressed  upon  the 
system  as  a whole,  and  in  the  same  plane  as  U,  then  the  resultant 
angular  velocity  is  the  algebraic  sum  of  the  two.  The  resultant 
frequency  will  also  be  the  algebraic  sum  ; and  since  is  to  be 
regarded  in  both  senses,  the  resultant  motion  will  have  a double 
frequency  of  N 4-  n and  N — n. 

Viewed  dynamically  the  equations  show  what  forces  are  neces- 
sary to  produce  the  above  changes.  If  the  ion  rotate  with  an 
angular  velocity  il,  and  the  orbit  itself  rotate  about  an  axis  passing 
through  the  center  of  force,  and  having  a direction  (Imii)  with  an 
angular  velocity  to,  then  the  component  velocities  referred  to  the 
rotating  orbit  are 


_ dx 

U ~ dt  ~ 

_dl__ 
V~  dt~ 

dz 

W=~di== 


1 

— tony  -f-  tomz 

— tolz  -f  tonx  I 

— to  nix- f-  toly 


(4) 


The  component  accelerations  are 
du 


I 


= — tonv  -f  tomw 


— — tolw  + tonu  [ 


dt 
dv 
dt 
dw 

— 77  = — to  mu  4-  tolv 
at  j 


(5) 


Expanding  equation  (5)  from  equation  (4)  the  acceleration  along 
the  axis  of  X is 


du  d2x 
dt  ~ dt  'L 


dy 


dz 


ton  -7-  + tom  —r~  — ton 
dt  dt 


ly) 


-j-  tom  | 


dz_ 

dt 


tomx  -f  to 


which,  by  adding  the  term  tol  (to  lx  — to  lx)  and  remembering  that 
/2  -f  //z2  + n2  — 1,  reduces  to 


JOHN  C.  SHEDD. 


[Vol.  IX. 


I 2 


(Px 

~dt2 


= - 2W  | 


dy 

n 

dt 


dk 


m -rr 


dt 


j — co2x  -f  co2 1 (lx  -J-  my  -j-  ns). 


Two  similar  expressions  give  the  acceleration  along  the  Y and  Z 
axes. 

The  total  acceleration  experienced  by  the  ion  is  then 


d2x 

dP 

-f  to2x  -f  co2 1 (lx  -j-  my  -f-  ns) 


**  + " (*  w - ” ii ) 


Z=  - iPz-dJL=  etc. 

dt 


(6) 


In  the  case  of  a magnetic  field  of  force,  if  the  axes  of  Z be  taken 
parallel  to  the  magnetic  field  then  (/,  m , ii)  become  equal  to  (o,o,  i) 
and  equations  (6)  reduce  to 


X = — Q2x  -|-  oj2x  -f  2 co 

dt 

F=  — Q2y  + (u2y  — 2(0  ~ 

Z=  - !Pz 


(7) 


Now  the  central  force  producing  the  original  rotation  is  I22x.  The 

dy 

perturbing  forces  are  then  represented  by  the  terms  co2x  + 2 co  -rr 


and 


to2y 


dx  . . . . . dy 

2 to  — • Examining  these  it  is  seen  that  2 to  ^ 


and  — 2 to 


dx 

dt 


are  the  X and  Y components  of  a force  2 tov  acting  perpendicularly 
to  v,  the  linear  velocity  of  the  ion.  If  then  a charged  ion  move  in  a 
magnetic  field  with  a velocity  v,  2 tov  is  the  force  it  would  experience 
due  to  the  magnetic  field.  The  terms  to2x  and  to2y  represent  cen- 
trifugal forces  due  to  the  impressed  velocity  to,  and  in  the  first 
approximation  may  be  neglected.  Finally,  if  K—  2 to,  the  above 
equations  become  identical  with  those  of  Lorentz. 

Equations  (6)  and  (7)  are  sufficiently  general  to  cover  all 
hitherto  observed  phenomena.  Thus  to  explain  the  case  where 


No.  i.] 


INTERFEROMETER  STUDY. 


I 3 


the  central  line  of  the  triplet  (line  2,  Fig.  2)  is  doubled,  it  is  only 
necessary  to  write  the  equation  for  Z in  the  form  Z—  A sin  Q t 
where  A is  a periodic  function  of  t of  the  form  A — a sin  nt.  Sub- 


stituting this  in  the  equation 


cPz_ 

dF 


= — Q2z  and  integrating  we  get 


Z—  a sin  nt  sin  Q t — 0/2  [cos  (Q  — n)  t — cos  (J2  -f  n)  t] , 
which  represents  two  vibrations  of  frequency 


(£2  dt  n)/27T.  (8) 

The  case  of  reversed  polarization  may  be  covered  by  supposing 
the  value  of  n to  be  such  as  to  separate  the  components  sufficiently 
to  place  them  outside  the  other  lines  of  the  triplet.  In  a similar 
manner  the  doubling  of  the  outer  members  of  the  triplet  may  be 
accounted  for  which  would  also  cover  the  case  of  a quartet  when 
viewed  parallel  to  the  magnetic  field.  In  this  way  all  the  various 
cases  of  multiple  lines  are  satisfactorily  explained. 

Reverting  to  equations  (2)  and  (3)  we  see  that  the  change  of 
period  of  the  ion  is  expressed  by  the  equation, 


27is/  m eH 

~K~'  2 Ks/m 


The  proportional  change  will  then  be 

T-  T'  eH  e HT 

T 2 Ks/m  m 4~ 

27i  s/  m 

since  K = — y~  • 

X )/ 

Finally,  since  T = — and  T9  — — where  v = velocity  of  light,  this- 


becomes 


/ - //  _ e H 

X2  m 47IZ>  * 


(9> 


Where  A = wave-length  of  the  spectral  line  for  zero  magnetic  field. 
/'  = wave-length  of  the  spectral  line  with  magnetic  field. 
v = velocity  of  light,  300,000,000  cm. 

H = intensity  of  magnetic  field  in  C.  G.  S.  units. 


14 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Equation  (9)  may  be  written 

e X — 4.7TV 

77 

and  it  is  seen  that  a measurement  of  the  change  of  wave-length  en- 
ables us  to  determine  the  ratio  of  the  ionic  charge  to  the  ionic  mass. 

Zeeman1  finds  for  this  ratio,  in  the  case  of  the  blue  line  of  Cd 
(A  = 4800)  24  x io6,  while  Preston2  has  found  that  this  value 
must  be  determined  separately  for  each  line,  but  that  a possible 
classification  of  lines  may  be  made  similar  to  the  chemical  classifi- 
cation of  Keyser  and  Runge.3 

Another  interesting  and  significant  observation  is  that  of  Ames4 
et.  alt.  who  find  that  in  the  case  of  some  iron  lines,  there  appears  to 
be  no  magnetic  effect,  while  those  lines  which  show  the  greatest 
magnetic  shift  are  the  ones  which  show  the  greatest  pressure  shifty 
aud  those  which  show  but  little  magnetic  shift  are  the  ones  of  little 
pressure5  shift. 

The  present  status  of  the  subject  may  be  summarized  as  follows  : 

1.  In  general,  spectral  lines  are  influenced  by  the  magnetic  field 
when  the  radiations  emanate  from  a source  of  light  in  the  magnetic 
field.6  The  magnetized  system  of  lines  is  symmetrical  with  respect 
to  the  original  unmagnetized  line.  New  spectral  lines  may  be  pro- 
duced by  the  magnetic  field.7 

2.  Viewed  parallel  to  the  magnetic  field  the  spectral  line  is,  in 
general,  doubled,  but  it  may  become  a single  or  multiple  line.  In 
the  case  of  a single  line  there  is  no  polarization  ;8  in  all  other  cases 
the  components  are  circularly  polarized ; the  shorter  wave-length  in 
the  direction  of  the  magnetizing  current,  the  longer  wave-length  in 
the  opposite  sense. 

3.  Viewed  in  a direction  perpendicular  to  the  magnetic  field  the 

1 Acad.  Amsterdam,  1S97-1898,  p.  260. 

2 Phil.  Mag.  (5),  45,  p.  337. 

3Wied.  Ann.,  Bd.  XLIII.,  p.  394,  1891. 

4 Astro.  Phys.  Jr.,  8,  p.  50,  1898. 

5 Astro.  Phys.  Jr.,  6,  p.  169,  1897. 

6 Astro.  Phys.  Jr.,  8,  p.  49. 

7 Astro.  Phys.  Jr.,  9,  p.  47. 

8 This  would  be  true  of  the  lines  seen  by  Becquerel  and  Ames,  showing  reversed 
po’arization,  when  viewed  parallel  to  the  magnetic  field. 


No.  i.] 


INTERFEROMETER  STUDY. 


J5 


outer  components  are  plane  polarized  so  that  their  vibrations  are  per- 
pendicular1 to  the  magnetic  field  ; the  inner  components  having  their 
vibrations  parallel  to  the  magnetic  field. 

4.  The  general  form  of  the  line  viewed  perpendicularly  to  the 
magnetic  field  is  a triplet,  but  it  is  sometimes  of  complex  structure. 

5.  The  amount  of  magnetic  action,  measured  in  change  of  period 
of  the  spectral  line,  is  not  a simple  function  of  the  wave-length  ; nor 
is  it  a constant  for  all  wave-lengths  ; nor  a constant  for  all  lines  of 
a given  substance  ; nor  is  it  a simple  function  of  wave-length  for 
the  lines  of  a given  substance. 

6.  The  magnetic  action  is  proportional  to  the  field  strength  (being 
limited,  however,  by  temperature  and  pressure) ; and  there  appears 
to  be  a possible  classification  following  the  chemical  classification  of 
Mendelejeff  and  of  Kayser  and  Runge. 

Part  Two.  Experimental  Work. 

Introductory  Note. — From  the  foregoing  survey  of  the  subject  we 
are  led  to  believe  that  a comprehensive  study  of  the  problem  con- 
sists essentially  of  two  parts  : 

1.  A qualitative  analysis  of  as  many  spectral  lines,  emanating 
from  as  many  different  substances  as  possible,  with  a classification 
according  to  the  type  of  line  produced. 

2.  Quantitative  measurements  of  the  change  of  wave-length  and 
of  the  ratio  e/m,  and  a classification  based  upon  these  measurements. 

All  spectral  lines  belonging  to  the  same  group  in  both  classifica- 
tions may  then  be  regarded  as  possessing  related  properties,  and 
the  ratio  elm  as  determined  from  such  a group  of  lines  should  have 
the  same  value. 

Such  a series  of  observations  would  as  Preston  remarks  “ afford 
a valuable  means  of  inquiry  into  the  so  far  hidden  nature  * * * of 
the  radiation  from  a luminous  body,  and  also  give  us  some  clearer 
insight  into  the  structure  of  matter  itself.” 

Outline  of  Work. — In  the  following  research  the  complete  study 
of  the  subject  was  not  attempted,  this  being  manifestly  too  great  a 
task  for  the  limited  time  available.  The  preliminary  ground  has, 
however,  been  cleared  and  a beginning  made. 

1 Exception,  see  Astro.  Phys.  Jr.,  8,  p.  50,  1898. 


JOHN  C.  SHEDD. 


[Vol.  IX. 


16 

The  work  was  subdivided  as  follows : 

Section  I.  A preliminary  survey  of  the  field  with  a view  of  de- 
termining the  conditions  limiting  the  observation  of  the  magnetic 
phenomenon. 

Section  II.  A comparison  of  the  ease  of  manipulation  and  range 
of  the  two  methods  outlined  above. 

Section  III.  To  ascertain  whether  the  magnetic  effect  is  radi- 
cally different  at  different  temperatures. 

Section  IV.  To  measure  the  magnetic  shift  of  as  many  lines 
as  the  time  available  would  permit ; studying,  also,  the  state  of 
polarization  of  the  components. 

Section  I.  A Preliminary  Survey  to  Determine  the  Conditions 
Limiting  the  Observation  of  the  Magnetic  Phenomena. 

Apparatus.  I.  Magnet. — This  was  of  the  usual  upright  type  ; 
the  base,  cores,  coils,  pole  heads,  and  cores  to  pole  heads  are  all 
separable ; the  base  and  cores  to  the  pole  heads  are  of  mild  steel, 
the  rest  of  the  magnetic  circuit  being  made  of  that  form  of  cast  iron 
known  as  Mitis  Metal.  An  elevation  of  the  magnet  is  shown  in 
Fig.  4,  drawn  to  yi  scale. 


The  cores  to  the  pole  heads  are  one  inch  in  diameter,  and  it  soon 
became  apparent  that  they,  together  with  the  mass  of  iron  behind 
them,  tended  to  lower  the  temperature  of  the  flame.  A second 


No.  i.] 


IN  / ERFER  OME  TER  S TUB  Y. 


17 


Fig.  5. 


pair  of  cores  was  made  of  the  form  shown  in  Fig.  5.  This  form 
also  concentrated  the  magnetic  field,  thereby  increasing  its  strength. 
A third  core  was  prepared  similar 
to  those  shown  in  Fig.  4,  with  a 
J^-inch  hole  throughout  its  length  ; 
this  core  was  for  use  in  viewing  the 
flame  parallel  to  the  magnetic  field. 

II.  The  Flame. — The  flame  of  a small  Bunsen  burner  was  first 
used  but  was  found  to  be  too  large.  A small  blast  lamp  was  then 
made  of  glass,  and  a foot  bellows  used.  This  gave  a small  conical 
jet  of  flame  about  3 inches  high  when  the  blast  was  inactive,  and 
about  ^ inch  high  with  the  blast.  This  form  of  lamp,  shown  in 
Fig.  6,  proved  highly  satisfactory.1 

To  color  the  flame  a strip  of  asbestos  wick  supported  by  platinum 
wire  was  first  used.  A bead  of  fused  sodium  carbonate  was  also 


h^GAS 


used,  and,  finally,  a rod  of 
sodium  glass  was  adopted 
as  needing  the  least  atten- 
tion. When  a very  bright 
flame  is  used  recourse  to 
the  fused  bead  may  be  had, 
but  such  a flame  generally 
gives  rise  to  spontaneous 
reversals.  In  the  latter  ex- 
periments the  glass  lamp 
was  set  aside  for  the  burner 
of  an  oxy-hydrogen  lamp, 
and  the  foot-blast  was  re- 
placed by  an  oxygen  tank. 

III.  Dispersion  Apparatus . — The  first  trials  were  made  with  a 
plane  grating  spectroscope  ; the  higher  spectra  were  especially  dim. 
No  success  was  attained  with  this  apparatus.  Next  a Rowland 
concave  grating  of  14,436  lines  to  the  inch  and’ five  feet  focal  dis- 
tance was  mounted  at  one  end  of  the  table,  the  magnet  being  at  the 
other  end.  The  spectrum  was  viewed  with  a telescope.  It  was 

1 In  time  the  glass  about  the  opening  cracks  away  but  the  whole  lamp  is  easy  of  con- 
struction. 


Fig.  6. 


i8 


JOHN  C.  SHEDD. 


[VOL.  IX. 


with  this  mounting  and  with  an  oxygen  gas  flame  that  the  first  re- 
sults confirmatory  of  Zeeman’s  work  were  obtained. 

The  arrangement  of  apparatus  is  shown  in  Fig.  7.  The  lines 
Dx  D2  were  very  sharp  and  bright.  When  the  current  was  turned 


Fig.  7. 


on  each  line  grew  broad  and  after  a few  seconds  became  distinctly 
double,  a sharp  dark  line  separating  the  components  throughout 
their  whole  length.  The  lines  were  visibly  brightened,  as  was  also 
the  whole  flame. 

When  the  current  was  interrupted  the  lines  appeared  to  collapse, 
and  after  a second  or  so  would  again  become  sharp.  Frequently 
the  doubling  of  the  lines  would  persist  for  a few  seconds  after  the 
current  was  broken  and  the  dark  dividing  line  could  be  seen, 
though  narrower  than  with  the  full  field. 

For  the  doubling  a field  strength  of  about  18,000  C.  G.  S.  units 
was  used,  with  weaker  fields  only  a broadening  of  the  line  could  be 
observed. 

Polarization. — With  zero  field  the  light  was  found  to  be  slightly 
polarized  by  reflection  from  the  grating.  The  polarization  with 

full  field  is  represented  in  Fig.  8,  the  ac- 
celerated components  Dx,  D2 2 harmon- 
ize in  their  sense  of  rotation  with  the 
magnetizing  current.  The  \ plate 
reduces  the  circular  polarization  to  plane 
polarization. 

Viewing  the  flame  in  a plane  perpen- 
dicular to  the  magnetic  field  the  pole  cores  shown  in  Fig.  5 were 
used.  The  tripling  can  best  be  observed  by  means  of  the  nicol  ; 
without  the  nicol  the  tripling  could  be  faintly  seen,  but  the  strength 
of  field  necessary  and  the  instrumental  difficulties  present  make  the 


No.  i.J 


INTERFEROMETER  STUDY. 


19 


observation  of  the  phenomena  far  from  satisfactory.  The  planes'  of 
vibration  of  the  components  are  shown  in  Fig.  9.  As  no  quarter 
wave  plate  is  used  this  is  the  state  of  vibration  in  the  ray  of  light 
itself. 

It  being  apparent  that  no  measurements  could  be  taken  without 
resorting  to  photography,  the  study  was  concluded  at  this  point 
and  work  with  the  interferometer  begun. 

Summary.  The  following  points  may  be  noted  as  covering  the  first 
point  aimed  at  in  the  work  : 

1.  The  magnetic  separation  of  the  sodium  lines  DY  Dv  as  given 
by  a naked  flame,  cannot  be  distinctly  observed  at  the  temperature 
of  the  Bunsen  flame,  nor  of  the  air-blast  flame,  nor  even  at  the  tem- 
perature of  the  oxygen  gas  flame,  unless  precautions  are  taken 
against  spontaneous  reversals. 

2.  The  phenomenon  can  be  much  more  satisfactorily  observed 
parallel  to  the  magnetic  field  when  perpendicular  to  it,  as  the 
strength  of  field  necessary  to  produce  a pure  (or  visual)  triplet  is 
twice  that  necessary  to  produce  the  doublet. 

3.  There  is  a very  perceptible  time  lag  both  when  the  magnet 
is  excited  and  when  the  current  is  broken,  during  which  period  the 
lines  show  an  inertia  effect.  This  lag  does  not  seem  to  be  wholly 
due  to  the  self-induction  of  the  fields,  but  may  be  partially  vis- 
ual and  partially  ionic. 

4.  A field  ^strength  of  at  least  15,000  C.  G.  S.  units  seems  to  be 
necessary  for  satisfactory  observation. 

5.  Spectra  above  the  second  order  are  too  faint  for  good  effects. 


Part  Two.  The  Interferometer  Method. 

Professor  Michelson  1 has  shown  the  peculiar  adaptability  of  the 
interferometer  for  the  class  of  research  here  considered . 

The  instrument  used  in  the  present  case  was  received  March  i, 
1898,  and  has  proved  highly  satisfactory  for  the  work  under- 
taken. 

The  instrumental  difficulties  attending  the  use  of  the  Interferom- 
eter are  such  as  perhaps  to  warrant  a word  about  them.  These 
difficulties  arise  from  two  sources  : (1)  defective  workmanship  in  the 
instrument,  and  (2)  those  due  to  the  observer.  These  will  be  con- 
sidered in  order  : (1)  the  most  serious  error  that  can  be  present  is 
a lack  of  parallelism  in  the  ways  along  which  the  mirror  carriage 
moves.  This  is  generally  manifested  by  a shifting  of  the  system  of 
fringes  in  the  field  of  view,  and  a more  rapid  loss  of  visibility  on 
the  part  of  the  fringes  than  should  take  place.2  A second  defect  is 
sometimes  present  in  the  optical  quality  of  the  glass,  due  either  to  a 
defective  surface,  or  to  internal  stress  in  the  glass.  This  defect  is 
manifested  by  a distortion  of  the  fringes  which  under^ proper  adjust- 
ment should  appear  as  circles.  A third  defect,  easily  overlooked, 
is  in  the  silvering  of  the  surfaces.  The  two  end  mirrors  (see  Fig. 
10)  M'  M"  should  be  heavily  silvered,  and  brightly  polished.  - 
The  half-silvered  surface  (Tf),  however,  should  have  such  a film 
as  will  transmit  and  reflect  equal  amounts  of  light.  If  this  be  true, 


1 Astro.  Phys.  Jr.,  6,  p.  48,  also  7,  p.  131. 

2 Phil.  Mag.  (5),  34  p.  286. 


No.  2.]  INTERFEROMETER  STUDY.  2 1 

50  jo  of  the  incident  light  is  effective  in  the  two  interfering  beams, 
and  the  fringes  are  of  maximum  brightness.  There  is  no  direct 
method  of  estimating  this  defect,  and  experience  is  the  only  proper 
guide.  (2)  A very  considerable  amount  of  patience  is  necessary  to 
eliminate  or  minimize  personal  errors.  Fringes  are  often  obtained 
which  appear  satisfactory,  but  from  a lack  of  exact  adjustment  have 
a variable  focus,  are  distorted,  and  rapidly  fade  out  as  the  movable 
mirror  recedes.  If  the  instrument  be  in  adjustment  the  fringes 
appear  as  concentric  circles,  and  Michelson  has  shown  1 that  if  the 
incident  light  be  parallel  then  the  fringes  are  at  infinity.2  If,  how- 
ever, the  light  be  not  parallel,  then  the  fringes  are  in  front  of  the 
interfering  surface  (M,  Fig.  10),  upon  it  or  behind  it,  according  as 
A is  greater  than,  equal  to,  or  less  than  zero,  where  A is  the  dif- 
ference in  the  paths  traversed  by  the  two  beams  of  light.  In  gen- 
eral it  may  be  said  that  the  incident  beam  is  not  parallel,  and 
hence,  that  the  fringes  recede  as  A increases.  If  a telescope  is 
used  to  view  the  fringes,  the  focus  will  have  to  be  altered  slightly, 
and  must  soon  be  adjusted  for  parallel  light.  If,  on  the  other  hand, 
the  naked  eye  is  used,  there  is  a tendency  to  focus  the  eye  upon 
the  mirror,  instead  of  upon  the  fringes.  Hence  an  observer  is  apt 
to  lose  sight  of  the  fringes  altogether,  or  to  view  them  out  of  focus 
unless  considerable  care  is  exercised  upon  this  point.  The  real 
work  of  the  interferometer  consists  in  an  eye  estimate  of  the  visibility 
of  the  fringes  as  A is  increased.  There  is  a manifest  liability  to  error 
in  these  eye  estimates,  unless  a comparison  set  of  fringes  of  known 
intensity  is  available  ; but  the  process  in  such  a case  is  not  only 
tedious3  but  adds  to  the  amount  of  apparatus  to  be  looked  after. 
On  the  whole,  it  is  more  practicable,  and  perhaps,  as  reliable  to  de- 
pend upon  the  training  of  the  eye  that  comes  from  long  practice,  and 
in  testing  the  eye  from  time  to  time  by  means  of  comparison  fringes. 
However,  even  after  a correction 4 curve  is  obtained  and  applied, 
there  is  room  for  a considerable  margin  of  personal  error,  which  is, 
perhaps,  the  chief  drawback  to  the  method  as  a whole. 

1 Phil.  Mag.  (5),  13,  p.  239,  1882. 

2 Except  when  A =°>  when  the  fringes  are  on  the  mirror. 

3 Astro.  Phys.  Jr.,  7,  p.  133. 

4 Phil.  Mag.  (5),  34,  p.  283.  Such  a curve  is  shown  in  Fig.  14. 


22 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Adjusting  the  Interferometer} 

The  general  disposition  of  apparatus  for  the  present  work  is  shown 
in  Fig.  io. 


JT  T M' 


Fig.  10. 


M,  M',  M//.  Interferometer  mirrors. 

Q.  X ^ plate. 

N.  Nicol. 

S.  Slit. 

E.  Eye  or  telescope. 

First  Adjustment.  To  Find  the  Fringes. — The  mirror  Mn  is  set 
so  as  to  be  within  two  or  three  mm.  of  the  zero  of  the  scale,  and 
a Bunsen  flame1 2  colored  with  a piece  of  sodium  glass  is  so  set  with 
reference  to  the  lens  C that  the  field  of  the  mirrors  is  uniformly 
bright.  Then  a pin,  or  bit  of  glass  fiber,  is  fastened  by  a bit  of  wax 
onto  the  lens,  so  that  its  image  is  in  the  center  of  the  field.  It  is 
preferable  to  have  the  image  lie  obliquely  across  the  field,  not  hori- 
zontally or  vertically.  The  mirror  M'  is  then  adjusted  until  the 
two  images  of  the  pin,  made  by  the  two  mirrors,  M and  Mf , are  super- 
posed. When  the  images  lie  obliquely  the  movements  of  the  adj  usting 
screws  can  be  easily  followed.  When  the  adjustment  is  very  close, 
if  the  room  is  partially  darkened,  and  the  eye  focused  for  a point 
behind  the  mirror,  the  fringes  should  appear  as  fine  lines  covering 
the  field.  The  eye  should  now  be  focused  on  these  fringes,  and 

1 The  Adjustments  of  the  Interferometer  are  discussed  at  length  by  Professor  Wads- 
worth in  Phys.  Rev.,  Vol.  4,  p 480.  See  also  p.  400  of  same  vol. 

2 The  Bunsen  flame  occupies  the  position  of  slit  S in  Fig.  10,  the  rest  of  the  system  to 
^he  right  of  S being  absent. 


No.  2.] 


INTERFER OME  TER  STUD  Y 


23 


the  screws  adjusted  so  that  the  fringes  grow  broader  and  more  dis- 
tinct. The  final  adjustment  is  reached  when  they  appear  as  con- 
centric circles,  which  do  not  change  in  appearance,  when  the  eye  is 
shifted  in  any  direction.  Under  these  conditions  the  two  mirrors, 
M and  M",  are  optically  parallel  to  each  other. 

Second  Adjustment.  To  Find  Zero  of  the  Instrument. — The  zero 
of  the  instrument  is  the  position  of  zero  difference  of  path  between 
the  two  interfering  beams  of  light.  It  is  found  by  obtaining  fringes 
with  white  light  and  adjusting  on  the  central  black  fringe  of  this 
system.  To  obtain  fringes  with  white  light  the  mirror  Mf  should 
first  be  adjusted  so  as  to  give  the  fringes  as  vertical  lines  of  small 
curvature.  If  now  the  mirror  M " be  moved  back  and  forth  by  slow 
motion,  the  fringes  will  be  seen  to  change  curvature  at  one  region. 
The  exact  point  of  this  change  of  curvature  is  the  point  sought. 
Having  adjusted  approximately  for  this,  a candle  flame  should  be 
set  before  the  Bunsen  flame,  giving  the  white  image  of  the  candle 
flame  with  a background  of  sodium  fringes.  The  mirror  M"  may 
now  be  slowly  moved  by  means  of  the  tangent  screw,  the  motion 
being  followed  by  the  eye,  in  the  progression  of  the  (faint)  sodium 
fringes.  When  the  zero  point  is  approached  brilliant  chromatic 
fringes,  from  ten  to  twenty  in  number,  will  appear  in  the  image  of 
the  candle  flame.  If  now  the  candle  be  removed,  and  the  air  cut 
off  from  the  Bunsen  flame  the  chromatic  fringes  are  very  distinct  and 
brilliant. 

The  zero  point  will  shift  a little  with  change  of  temperature,  and 
still  more  by  a change  in  the  distance  of  the  mirror  M'  from  M due 
to  difference  in  the  manipulation  of  the  adjusting  screws.  The  zero 
should  therefore  be  observed  before  each  set  of  observations. 


Visibility  Curves. 

The  theory  of  the  visibility  curve  has  been  given  by  Michelson, 
and  its  adaptation  shown  to  problems  where  the  distribution  of  light 
as  a source  of  radiation  is  to  be  determined. 

The  following  curves  were  taken  using  sodium  light  as  source  of 
illumination,  and  viewing  the  light  first  perpendicular  to  the  field, 
and  than  parallel  to  it. 


24 


JOHN  C.  SHEDD. 


[Vol.  IX. 


I.  Perpendicular  to  the  Field.  The  curves  taken  are  shown  in 
Fig.  ii , A,  and  the  corresponding  distribution  of  light  in  Fig.  1 1,  B1. 


In  this  curve  the  nicol  prism  was  not  used,  and  the  loss  of  light 
by  the  central  component  (which  is  known  to  be  present),  is  such 
as  to  render  the  line  an  apparent  double.2  In  the  curves  B,  in  those 
marked  B,  C}  D the  main  components  seem  accompanied  by  small 
companions  some  having  negative  ordinates.  Now  since  this  would 
indicate  negative  intensities,  they  indicate  errors  in  observing  the 
curves  A.  The  analysis  of  the  curves  A then  furnish  a valuable 
check  upon  their  correctness. 

II.  Light  Viewed  Parallel  to  the  Magnetic  Field.  Curves  were 
taken  for  this  position  both  with  and  without  the  nicol.  The  re- 
sults are  shown  on  Figs.  12  and  13.  These  are  substantially  iden- 
tical and  show  first  a broadening  and  then  a doubling  of  the  spectral 
lines.  The  presence  of  the  negative  ordinates  is  also  seen  and  yet 
the  substantial  results  are  clear. 

1 Acknowledgment  is  due  to  Professor  Michelson  who  kindly  facilitated  the  analysis 
of  these  curves  on  the  Harmonic  Analyzer  at  the  University  of  Chicago. 

2 See  part  I,  also  Michelson,  Astro.  Phys.  Jr.,  6,  p.  49. 


No.  2.] 


INTERFER  OME  TER  S TUD  Y. 


25 


Fig.  12. 


\' 
V V 

A 

Na 

li 

FLA 

MAGN. 

ME  yiE 

ETIC  FIEI 

LWED 
-D  1 

CURV 

A- 

E H 

0. 

V 

V 

\ \ 

— - 

\ 

f4 

A) 

<1S  Vi|  A PLATE  VERTIC/j 

“ NICOL  | | “ 

kL 

Ef- 

c- 

— 300 

— 770 

0 

0 

\ 

c' 

B 

\ 

v 

LINES  PI 

RESENT  1 

31  D1  °2 

mm. 

A 

i i 

5 1 

0 1 

2 1 

1 1 

6 

S ;2 

d 

0 2 

12  2 

1 20 

-28=3 

0 3 

2- 

\ 

V 

A 

B DISTRIBUTION 

M 

CURVES 

l 

'a'  ~ e 

i ~ c 

Fig.  13. 


26 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Having  analyzed  these  curves  a comparison  curve  was  then  taken 
to  observe  what  correction  was  necessary  to  reduce  the  eye  esti- 
mates shown  in  the  full  line  curves  of  Figs.  11,12  and  13.  The  re- 
sulting curve  is  shown  in  Fig.  14  and  the  corrected  visibility  curves 
are  shown  in  the  dotted  curves  of  Figs.  12  and  13.  These  corrected 
curves  were  not  analyzed,  but  their  distribution  curves  would  be 
more  nearly  correct  than  those  shown.  We  have  then  in  the  curves 
shown  all  observational  errors  present ; the  result,  though  not  satis- 
factory, is  by  no  means  bad.  This  form  of  error  is  very  hard  to 
avoid  and  long  practice  alone  can  eliminate  it. 


100 


Fig.  14. 


Polarization. — The  interferometer  was  found  to  be  capable  of 
showing  the  state  of  polarization  in  the  most  elegant  manner,  not 
only  identifying  the  plane  of  polarization,  but  also  immediately 
identifying  the  accelerated  from  the  retarded  ray. 

Thus  with  the  axis  of  the  X plate  vertical  the  nicol  was  ro- 
tated and  the  action  and  character  of  the  fringes  observed.  The 
following  were  the  results  obtained  : 


No.  2.] 


INTERFEROMETER  STUDY. 


27 


Table  II. 


Position  of  Axis 
of  Nicol. 

Action  of  Fringes  on 
Magnetization. 

Character  of  Central  Fringe 
with  Magnetic  Field. 

0°  Vertical. 

Become  hazy. 

Hazy. 

4-  45° 

Expand  and  remain  distinct. 

Distinct,  dark. 

-(-  90°  Horizontal. 

Become  hazy. 

Hazy. 

135° 

Contract  and  remain  dktinct. 

Distinct,  light. 

180°  Vertical. 

Become  hazy. 

Hazy. 

—135° 

Expand  and  remain  distinct. 

Distinct,  dark. 

— 90°  Horizontal. 

Become  hazy. 

Hazy. 

1 

4* 

Ln 

0 

Contract  and  remain  distinct. 

Distinct,  light. 

0°  Vertical. 

Become  hazy. 

Hazy. 

It  is  evident  that  the  haziness  at  o°  db  90°  and  180°  is  due  to  the 
simultaneous  contracting  and  expanding  of  the  fringes.  Further 
analysis  shows  us  that  when  the 
fringes  expand  the  retarded  compon- 
ent is  present  and  when  the  fringes 
contract  the  accelerated  (or  shorter 
wave-length)  component  is  present. 

This  is  shown  in  Fig.  15.  Unpol- 
arized light  is  represented  by  the  su- 
perposition of  the  two  perpendicular 
planes  of  vibration. 

From  this  general  survey  of  the  in- 
terferometer we  derive  the  following- 
generalizations  : 

1 . The  interferometer  is  capable  of  showing  the  magnetic  effect 
for  field  strengths  below  1,000  C.  G.  S. 

2.  The  visibility  curves,  even  under  unfavorable  circumstances, 
show  clearly  the  general  character  of  the  magnetic  effect  and  when 
checked  through  a long  series,  by  means  of  an  harmonic  analyzer, 
furnish  an  incomparable  method  of  analysis. 

3.  When  unaccompanied  by  such  a check  errors  are  not  readily 
eliminated  and  for  quantitative  measurements  of  change  in  wave- 
length another  use  of  the  interferometer  furnishes  a better  method. 

Inasmuch  as  the  visibility  curve  analysis  was,  not  under  the  cir- 
cumstances, entirely  available,  attention  was  tufned  to  the  develop- 
ment of  point  3 above  and  carried  out  as  follows  : 


Fig.  15. 

Axis  of  ^ 1 plate  vertical, 
Nicol  rotated. 


28 


JOHN  C.  SHEDD. 


[VOL.  IX. 


Measurements  of  Change  of  Wave-length. 

Method. — Professor  Michelson  1 in  determining  the  difference  in 
wave-length  between  the  components  of  the  magnetic  line  makes 
use  of  what  he  calls  the  “period  of  coincidence  due  to  doubling.” 
This  may  be  defined  as  follows  : Consider  two  systems  of  fringes 
produced  by  light  sources  differing  but  little  in  wave-length.  Then 
the  residtant  system  due  to  both  sources,  will  be  the  resultant  of 
the  overlapping  of  the  component  systems,  and  at  certain  points 
destructive  interference  will  take  place,  and  at  others  reinforcement. 
The  fringes  will  then  run  through  a series  of  maxima  and  minima 
as  A is  increased.  The  distance  from  maximum  to  maximum  is  the 
“period.”  If  A be  measured  from  “zero”  to  the  first  point  of  re- 
inforcement, then  the  following  equation  holds 

Ap  = M=(N+  i)V  (II) 

where  N is  the  total  number  of  fringes  ; and  A and  A are  measured 
in  mm.  In  the  case  of  the  lines  Dl  D2  these  maxima  occur  every 
988  fringes  and  AJ;  = 0.58242  mm.  In  the  case,  however,  of  the 
magnetic  shift  the  difference  of  wave-length  is  so  small  that  Michel- 
son finds  the  values  of  A to  range  from  58  mm.  to  14  mm.  as  the 
magnetic  field  increases  to  4,000  C.  G.  S.  units.  The  same  method 
is  also  described  by  Perot  and  Fabry 2 and  is  especially  applicable 
where  the  difference  of  wave-length  is  sufficient  to  give  a differ- 
ence of  color  to  the  two  sets  of  fringes. 

When  applied  to  the  magnetic  components  the  method  is  open  to 
improvement.  In  the  first  place  the  fringes,  at  a point  where  A = 
50,  or  even  30,  are  so  narrow  and  frequently  so  faint  that  an  accu- 
rate determination  of  points  of  reinforcement  (i.  e.y  where  both  sys- 
tems agree  in  phase)  is  not  easy.  Secondly,  since  the  magnetized 
and  unmagnetized  fringes  cannot  be  simultaneously  observed,  the 
above  determination  consists  in  finding  the  value  of  A at  which  the 
closing  of  the  magnet  shifts  the  system  the  double  width  of  one 
fringe.  This  determination  for  high  values  of  A is  not  as  easy  as 
might  be  wished. 

1 Astro.  Phys.  Jr.,  6,  p.  50. 

2 Astro.  Phys.  Jr.,  Feb.,  1899.  See  Appendix  III. 


No.  2.] 


INTERFEROMETER  STUDY, \ 


29 


There  are,  however,  two  possible  modifications.  If  the  nicol  be 
not  used,  it  has  been  seen  that  the  magnetized  components  draw 
apart,  one  being  retarded  and  the  other  accelerated.  The  corre- 
sponding effect  on  the  fringes  is  to  cause  the  original  system  simul- 
taneously to  contract  and  expand.  If  now  the  point  is  found  at 
which  the  two  systems  of  fringes  are  tangent  to  each  other,  each 
fringe  will  have  shifted  its  own  width,  showing  that  the  acceler- 
ated system  is  period  in  advance  of  the  retarded  one,  or  ^ 
period  removed  from  the  original  system.  We  then  have  the 
equation 

A m,-M-(N+%)X’.  (.2) 

The  second  modification  consists  in  using  the  nicol  and  in  thus 
quenching  one  of  the  component  systems.  Now  concentrate  the 
attention  upon  one  fringe,  either  adjusting  a pointer  (fastened  to 
lens  c,  Fig.  10)  to  its  edge,  setting  the  fringe  tangent  to  a line  drawn 
on  the  mirror  M,  or  by  merely  observing  the  central  fringe  of  the 
system.  Let  it  be  supposed  that  the  central  fringe  is  dark,  and  the 
field  unmagnetized.  On  turning  on  the  current  the  system  contracts, 
and  for  a certain  value  of  A will  be  the  exact  complement  of  the 
first,  the  center  being  now  light  instead  of  dark. 

Under  these  conditions  the  fringes  have  shifted  over  their  own 
width,  and  the  following  equation  is  obtained. 

a ,Ap  = m = {N+y2)i'.  (13) 

Still  a third  modification  would  be  as  follows  : Let  the  field  re- 
main magnetized  and  set  the  fringe,  with  the  nicol  in  a given  posi- 
tion, tangent  to  the  fixed  line.  Then  let  the  nicol  be  rotated  90°,  so 
' as  to  bring  the  other  system  of  fringes  into  view  ; then  that  value 
of  A which  renders  the  line  a common  tangent  to  both  systems 
gives  a difference  of  period  such  that 

A =Mi==(N+  i)X2 

where  ^ and  X2  are  the  component  lines.  Also  since  Xx  and  X2  lie 
symmetrically  with  respect  to  X (the  unmagnetizedTine)  we  have 

A = NXx  = [N  + y2)X=(N+  i)X2 


JOHN  C.  SHEDD. 


[Vol.  IX. 


30 

or,  putting  the  equation  in  terms  of  X, 

a =m=(N±y2y  (14) 

which  is  identical  with  equation  (13). 

Since  in  the  above  equations  N is  the  total  number  of  fringes 
passed  over  from  the  position  A = o,  some  means  must  be  had  of 
knowing  its  value.  This  is  obtained  very  simply  as  follows  : If  X be 
given  in  mm.  then  i/X  equals  the  number  of  fringes  per  mm., 
and  N — A /^,  A being  measured  in  mm.  Substituting  this  value 
in  the  above  equations  and  solving  for  X—X',  the  following  equa- 
tions are  obtained 


For  A equation  (11) 

X2 

(11') 

i X' 

~ _ A 

~ A 

For  A^  equation  (12) 

, ,,  //' 

X2 

x - xr  = - — 

— 

(12') 

4A 

4A 

For  A y2p  equation  (13) 

xx' 

X2 

(13') 

X — A'  = r 

2A 

~ 2 a 

For  a given,  value  of  X — X'  we  have  the  relation 

^P  = 4./tp  = 2AHp.  (16) 

This  last  equation  would  indicate  that  the  second  method  given 
by  equation  (12')  would  be  the  most  accurate  since  it  gives  the 
smallest  values  of  A and  hence  the  widest  and  brightest  fringes. 
It  must,  however,  be  noticed  that  the  condition  expressed  by 
equation  (12)  is  that  the  two  systems  of  fringes  shall  differ  by  y 
period  and  hence  that  the  dark  rings  of  one  system  shall  coincide 
with  the  light  rings  of  the  other.  Hence  the  field  will  be  uni- 
formly illuminated  and  the  fringes  disappear.  In  practice  there  is 
found  to  be  a region  over  which  the  fringes  are  blurred  and  the  ex- 
act point  of  extinction  is  difficult  to  determine. 

On  the  other  hand,  in  the  third  method  the  fringes  are  always 
sharp  and  for  values  of  H above  2,000  C.  G.  S.  units  the  values 
of  A are  small  enough  to  render  the  fringes  sufficiently  wide  for 
satisfactory  observation. 


No.  2.] 


INTERFER  OR/E  TER  S TUD  V. 


3* 


With  a little  practice  the  method  of  observing  the  central  fringe 
was  found  to  give  concordant  readings  and  hence  was  used  in  prefer- 
ence to  setting  the  fringe  tangent  to  a line. 

A comparison  of  the  three  methods  is  given  by  equation  (16). 

j ^ 

Still  further  light  is  given  by  substituting  the  value  of  — from 

equations  (13)  in  equations  (9). 

Making  this  substitution  we  get 


1 st  method. 

2d 

3d 


m 


H A =4  xv  — 


m 


H A = xv  — 


H A 


m 

2TC  V — 

e 


(n") 

(12") 

(13") 


07) 


If  m/e  be  regarded  as  constant,  these  equations  all  represent  rec- 
tangular hyperbolae  asymptotic  to  H and  A taken  as  axes. 

It  is  thus  seen  that  curves  expressing  the  relation  of  H and  A 
furnish  a ready  means  of  comparison  both  by  equations  (16),  and 
by  their  form  as  indicated  by  equations  (17).  Such  a set  of  com- 
parison curves  was  taken  with  an  oxygen  gas  flame  colored  with 
sodium.  In  Table  III.  the  values  of  H A for  different  parts  of  the 
curve  are  given,  and  the  curves  themselves  are  given  in  Fig.  16. 


Table  III. 


Method  II.  Method  III.  Method  I. 


I Z1 

A 

HA 

(curve  A.) 

A 

HA 

(curve  B.) 

A 

HA 

(curve  C.) 

2,300 

15.4 

354 

21.4 

492 

13.1 

300 

3,000 

12.0 

360 

16.7 

500 

12.0 

360 

4,000 

9.0 

360 

13.3 

530 

11.0 

440 

5,000 

7.3 

365 

10.5 

526 

10.0 

500 

6,000 

6.2 

370 

8.2 

490 

9.0 

540 

7,000 

5.5 

385 

6.8 

475 

8.2 

575 

8,000 

4.8 

384 

5.8 

465 

7.4 

590 

9,000 

4.3 

386 

5.2 

470 

6.7 

602 

10,000 

3.8 

380 

4.8 

480 

6.1 

610 

11,000 

3.5 

386 

4.5 

496 

5.6 

615 

12,000 

3.4 

408 

4.4 

528 

5.3 

635 

1 H throughout  these  experiments  was  determined  by  the  balistic  method. 


32 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Curves  A and  B corresponding  to  Methods  II.  and  III.  show  a 
close  correspondence  with  equation  (17)  while  curve  C is  manifestly 
unreliable. 


Comparing  curves  A and  B we  see  that  equation  (16)  is  not  ful- 
filled, and  hence  that  either  one  or  both  curves  are  displaced.  From 
the  fact  already  pointed  out  as  to  the  difficulty  of  making  settings  for 
curve  A we  have  no  hesitation  in  concluding  that  the  readings  for 
this  cure  are  uniformly  too  high.1  Assuming  that  curve  B is  cor- 
rect, the  dotted  curve  D gives  the  corresponding  position  for 
curve  A. 

Section  III.  Comparison  of  Magnetic  Shift  at  Different 

Temperatures. 

To  investigate  this  the  sodium  flame  was  used  and  reversal  curves 
taken,  by  Method  III.,  under  the  following  conditions  : I.  Bunsen 
flame,  II.  Oxy-gas  flame,  III.  Vacuum  tube. 

1 It  is  not  to  be  supposed  that  curve  A represents  the  greatest  accuracy  attainable  by 
Method  II.  Continued  observation  would  undoubtedly  render  this  method  quite  avail- 
able. 


No.  2.] 


INTERFER  OME  TER  S TUD  Y 


n 'i 

OO 

The  light  was  viewed  parallel  to  the  magnetic  field  as  giving  the 
most  uniform  structure  of  line  (see  Fig.  3,  B),  and  a mica  plate 
used.  The  following  tables  are  selected  as  representative. 


Table  IV. 

Reversal  Curves.  Na  in  Bunsen  Flame. 


H 

C.  G.  S.  units. 

A mm. 

A — A'  A.  U. 

0,000 

00 

0.000 

3,000 

12.80 

0.135 

4,000 

11.65 

0.155 

5,000 

10.60 

0.168 

6,000 

9.60 

0.180 

7,000 

8.50 

0.205 

8,000 

7.70 

0.230 

9,000 

6.40 

0.274 

9,500 

5.40 

0.325 

9,600 

5.00 

0.350 

9,700 

4.90 

0.370 

9,800 

4.80 

0.375 

10,000 

4.70 

0.380 

11,000 

4.65 

0.380 

12,000 

4.65 

0.380 

13,000 

4.65 

0.380 

34 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Table  IV.  is  platted  in  Fig.  17.  The  part  of  the  curve  lying  be- 
tween 8,000  C.  G.  S.  and  11,000  was  checked  up  with  special  care 
and  the  knee  in  the  curve  verified.  For  all  readings  where  A is 
less  than  3 mm.  a small  change  in  A indicates  a large  change  in 
X — X',  and  hence  the  readings  become  less  sensitive.  The  departure 
of  the  reversal  curve  from  the  normal  hyperbolic  form  would  in- 
dicate, as  is  shown  in  curve  B , Fig.  17,  an  ionic  inertia  or  lag,  due 
either  to  low  temperature  or  high  density  of  the  gas,  or  to  both 
This  constraint  retards  the  change  in  wave-length  but  suddenly 
gives  way  at  a field  strength  of  about  9,500.  Above  a value  of 
H,  of  10,000  the  mean  wave-length  of  the  lines  do  not  seem 
further  to  change,  increase  in  H beyond  this  point  being  effective  in 
broadening  the  lines  rather  than  in  separating  them. 

In  curves  A and  B (Fig.  18)  a small  flame  2 cm.  high  was  used 
with  the  tip  of  the  inner  cone  opposite  the  aperture.  In  curve  C 
the  jet  was  15  cm.  high  and  the  base  of  the  flame  opposite  the 
opening.  The  flame  in  both  cases  was  colored  with  a bead  of  fused 
sodium  carbonate. 

Curve  C shows  the  same  “lag”  as  does  the  Bunsen  flame,  and 
the  maximum  value  of  X — X'  is  not  so  large.  In  curves  A and  B , 


Table  V.  See  Fig.  18. 

Reversal  Curves.  Na  in  Oxygen-gas  Flame. 


H 

mm. 

A b mm. 

A emm. 

U-a')a 

(*-*')* 

( A.+V  ) c 

0 

00 

00 

00 

0.000 

0.000 

0.000 

2,500 

17.20 

18.50 

19.40 

0.105 

0.097 

0.090 

3,000 

16.00 

17.15 

18.00 

0.111 

0.105 

0.100 

4,000 

13.30 

14.35 

15.35 

0.135 

0.125 

0.115 

5,000 

10.80 

11.70 

12.80 

0.164 

0.151 

0.140 

6,000 

8.25 

9.00 

10.70 

0.212 

0.197 

0.165 

7,000 

6.20 

6.95 

9.10 

0.285 

0.251 

0.197 

8,000 

5.10 

5.80 

7.80 

0.345 

0.304 

0.228 

9,000 

4.55 

5.10 

6.90 

0.390 

0.345 

0.254 

9,500 

4.40 

4.90 

6.50 

0.400 

0.360 

0.275 

10,000. 

4.30 

4.70 

6.00 

0.410 

0.375 

0.310 

10,500 

4.25 

4.50 

5.40 

0.420 

0.402 

0.334 

11,000 

4.15 

4.40 

5.15 

0.430 

0.402 

0.345 

12,000 

4.05 

4.40 

5.10 

0.442 

0.402 

0.345 

12,700 

4.00 

4.40 

5.10 

0.442 

0.402 

0.345 

No.  2.] 


INTER PER  OME  7ER  S TUD  Y 


35 


Fig.  18. 


which  are  at  a higher  temperature,  this  lag  is  overcome  at  a lower 
value  of  H,  and  the  maximum  value  of  / — //  is  larger. 


Table  VI.  See  Fig.  19. 

Reversal  Curve.  Na  in  Vacuum  Tube. 


H 

A mm. 

A — y A.  u. 

0 

00 

0.000 

5,000 

11.20 

0.160 

6,000 

9.50 

0.186 

7,000 

7.90 

0.225 

8,000 

6.30 

0.280 

9,000 

5.00 

0.352 

9,500 

4.60 

0.384 

10,000 

4.25 

0.415 

10,500 

4.10 

0.431 

11,000 

4.00 

0.441 

12,000 

4.00 

0.441 

13,000 

4.00 

0.441 

JOHN  C.  SHEDD. 


[Vol.  IX. 


Fig.  1 9 would  seem  to  show  that  at  the  temperature  of  the  vacuum 
tube  the  change  of  7 — //  is  nearly  proportional  to  H up  to  7,000  C. 
G.  S.,  at  which  point  the  “ lag  ” is  overcome.  At  1 1,000  the  maxi- 
mum for  X — is  reached. 

In  the  case  of  the  vacuum  tube,  the  tube 
was  arranged  as  shown  in  Fig.  20.  The 
brass  tube  T is  made  to  fit  over  the  pole 
heads  of  the  magnet,  and  is  capped  at  either 
end,  thus  forming  a metal  box.  Into  this 
box  the  vacuum  tube  is  placed,  being  held 
from  contact  with  the  metal  by  asbestos 
wool. 

A Bunsen  flame  is  so  supported  as  to 
play  horizontally  upon  the  tube.  Small  glass  tubes  enter  the  caps 
and  convey  wires  to  the  terminals  of  the  vacuum  tube  which  was 
energized  by  an  induction  coil  of  the  usual  type.  The  substance 
whose  spectrum  was  to  be  examined  was  placed  in  the  tube,  in  the 
form  of  filings,  and  the  tube  then  exhausted  to  between  1 mm.  and 
2 mm.  pressure.  The  brass  tube  was  then  heated  by  means  of  the 


No.  2.] 


INTERFEROMETER  STUDY. 


37 


Bunsen  flame,  until  the  spectral  lines  appeared  sharp  and  bright. 
In  some  cases  the  heating  was  pushed  too  far,  resulting  in  chemical 
action  on  the  glass. 

Upon  using  the  vacuum  tubes  it  was  found  that  the  glass  of  the 
tube  itself  acted  as  a retardation  plate,  adding  its  effect  to  that  of 
the  X mica  plate.  It  was  thus  found  impossible  to  produce  an 
exact  retardation  of  period.  With  reversed  positions  of  the  nicol 
prism,  however,  the  two  systems  of  fringes  were  found  to  be  clear 
and  sharp  so  that  the  arrangement  was  satisfactory  if  not  ideal.  The 
mica  plate  was  used  part  of  the  time,  and  part  of  the  time  discarded. 

In  the  preceding  curves  the  effect  of  pressure  cannot  be  separated 
from  the  temperature  effect.  The  combined  effect  is  what  is  ob- 
served. An  interesting  detail  yet  to  be  studied  is  the  case  where 
the  pressure  is  gradually  varied,  and  a pressure  curve  obtained. 

Summary. — From  a study  of  these  curves  we  draw  the  following 
conclusions  with  regard  to  the  effect  of  temperature. 

I.  At  the  temperature  of  the  Bunsen  flame  there  is  a distinct 
ionic  “lag”  or  constraint  which  is  overcome,  suddenly,  at  a field 
strength  of  9,500  C.  G.  S.  units. 

II.  This  ionic  lag  becomes  less  as  the  temperature  rises  and  is 
practically  absent  at  the  hottest  temperature  of  the  oxygen-gas  flame 
or  of  the  vacuum  tube. 

III.  The  change  in  wave-length  reaches  a maximum  value  depend- 
ing upon  the  temperature  (and  pressure);  the  maximum  is  reached 
at  about  1 1,000  C.  G.  S.  For  values  of  H above  this  the  magnetic 
effect  is  to  broaden  the  lines  and  not  to  separate  them  further.1 

This  latter  point  receives  support  from  Michelson’s  echelon  spec- 
troscope which  shows  the  sodium  lines  to  belong  to  type  II.  (See 

Fig.  3,  B. 

Note.  An  attempt  was  made  to  take  observations  witli  the  spark  gap.  A glass  tube  1 
mm.  bore  was  taken,  and  metallic  sodium  crowded  into  it  until  2 or  3 cm.  of  the  tube  at  the 
center  was  filled.  The  tube  was  then  cut  at  the  middle  of  the  sodium,  and  the  free  ends 
used  as  spark  terminals. 

On  turning  on  the  magnet  the  electro-dynamic  action  of  the  field  caused  a spark  to  fan 
out  showing  in  a very  pretty  manner  the  magnetic  equipotential  surfaces.  This  has  also 
been  observed  by  M.  Cornu.  (Astro.  Phys.  Jr.,  7,  p.  166,  note  2.) 

1 This  may  be  peculiar  to  the  sodium  lines,  and  be  due  to  the  simultaneous  presence  of 
the  lines  Dx  and  Dr  No  observations  have  yet  been  made  upon  them  separately,  the 
dispersion  necessary  to  isolate  them  being  too  great. 


JOHN  C.  SHEDD. 


[Vol.  IX. 


8 


Section  IV.  Measurements  of  Magnetic  Shift. 
Equation  (9)  may  be  put  into  the  form 


showing  that  for  a given  value  of  \ the  change  in  wave-length  should 
be  proportional  to  H provided  no  constraint  is  present.  We  have 
seen  that  in  the  case  of  sodium  (and  presumably  for  all  sub- 
stances) a constraint  is  present  at  low  temperatures,  but  that  it 
disappears  as  the  temperature  rises.  In  the  measurement  of  mag- 
netic shift  it  is  important  that  the  temperature  be  high,  and,  pre- 
ferably, that  the  pressure  be  low.  These  conditions  are  fulfilled  by 
the  vacuum  tube ; hence  it  was  used  as  a source  of  illumination. 

It  was  primarily  intended  to  make  an  extended  series  of  observa- 
tions upon  the  spectral  lines  of  different  substances,  but  the  difficul- 
ties encountered  of  breaking  tubes,  deposits  in  the  capillary,  and 
chemical  action  within  the  tube,  were  such  as  to  reduce  the  number 
of  satisfactory  observations  to  sodium,  zinc,  mercury  and  cadmium. 
Difficulty  was  also  rqet  with  in  getting  readings  upon  lines  lying 
near  either  end  of  the  visible  spectrum. 

The  following  are  the  data  taken : 

Sodium.  Yellow  lines  (DXD^. — The  probable  linear  relation 
of  / — /'  to  H can  be  derived  from  Fig.  21,  curve  B,  by  drawing  a 
line  from  the  origin  tangent  to  the  curve.  This  is  given  in  Fig.  24. 

Zinc.  Blue  line.  / = 4810.724.  (For  arrangement  of  appa- 
ratus, see  Fig.  10.) 

Table  VII. 


H.  C.  G.  S. 

A mm. 

hx  a 

4,000 

10.2 

408 

5,000 

8.30 

415 

6,000 

6.80 

408 

7,000 

5.80 

406 

8,000 

5.05 

404 

9,000 

4.40 

396 

9,500 

4.15 

394 

10,000 

3.95 

395 

10,500 

3.85 

405 

Aver.  =403. 


No.  2.] 


INTERFEROMETER  STUDY. 


39 


The  average  of  column  three  is  the  product  of  H x A of  the 
equivalent  hyperbola,  and  this,  divided  by  any  given  value  of  Hf 
gives  a corrected  value  for  A,  which  may  be  used  in  determining 
the  value  of  X — X' . In  this  way  a double  check  is  secured,  first,  in 
drawing  a smooth  curve  through  the  original  data,  and  thus  getting 
the  readings  of  column  two;  and  second,  in  “averaging”  these 
readings  in  the  manner  shown. 

In  Table  VII.  the  average  of  column  three  is  403  ; the  value  of  A 
corresponding  to  ff=  5,000,  is  8.06,  and  X — X'  is  0.144  A.  U. 
This  is  given  in  Fig.  21. 

Mercury. — Readings  were  obtained  on  the  yellow,  green  and  violet 
lines  as  follows  : 


Table  VIII. 


H 

Yellow  line. 

Green  line. 

Violet  line. 

K = 5790-49- 

A.  = 5460.97. 

A = 4358.56. 

A 

//A 

A 

HA 

A 

HA 

3,000 

20 

600 

4,000 

15. 

600 

12.0 

480 

5,000 

11.9 

595 

9.5 

475 

6,000 

9.85 

591 

8.0 

480 

7,000 

8.25 

578 

6.8 

476 

5.7 

399 

8,000 

7.00 

560 

5.8 

464 

4.8 

384 

9,000 

6.20 

558 

5.2 

468 

4.3 

387 

10,000 

5.70 

570 

4.8 

480 

3.8 

380 

11,000 

5.45 

560 

4.7 

517 

3.5 

385 

Aver.  = 579.  Aver.  = 480.  Aver.  = 387. 


The  magnetic  shift  for  H = 5,000  is,  yellow  line  o.  128  A.  U.,  green 
line,  0.155  A.  U.,  violet  line  0.120  A.  U. 

Lines  are  then  drawn  through  zero  and  this  value,  thus  giving 
the  magnetic  shift  for  any  value  of  A.  This  is  done  on  Fig.  22. 

Cadmium. — Readings  were  taken  on  the  red,  green  and  blue  lines 
as  shown  in  Table  IX.,  and  platted  on  Fig.  23. 


40 


JOHN  C.  SHEDD. 


[Vol.  IX. 


Table  IX. 


H 

Red  line. 

A =6438.9. 

Green  line. 

A = 5086.3. 

Blue 

A = , 

line. 

4800. 

A 

HX  A 

A 

HX  A 

A 

HX  A 

3,000 

. 

. 

15.8 

475 

13.9 

418 

4,000 

18.6 

745 

13. 

520 

10.7 

432 

5,000 

14.15 

708 

10.8 

540 

8.4 

420 

6,000 

11.4 

685 

9. 

540 

6.85 

411  . 

7,000 

9.8 

685 

7.8 

546 

5.9 

413 

8,000 

8.75 

700 

7. 

560 

5.3 

424 

9,000 

8. 

720 

6.6 

594 

• • 

• • 

Aver.  = 707. 


Aver.  = 539.  Aver.  = 420. 


The  magnetic  shift  for  H — 5,000  is  red  line,  0,131  A.  U.;  green 
line,  0.120  A.  U.;  blue  line,  0.137  A.  U. 

The  results  may  be  summarized  as  follows  : 


Table  X. 


Substance. 

Line. 

Magnetic  Shift  for 

H=  5000  H = 10.000 

A.  U. 

A.  u. 

Sodium.1 

Yellow  line  Dx. 

0.207 

0.414 

Mercury. 

Yellow  line. 

0.128 

0.256 

a 

Green  “ 

0.155 

0.310 

i 6 

Violet  “ 

0.120 

0.240 

Cadmium. 

Red 

0.131 

0.262 

<< 

Green  “ 

0.120 

0.240 

< i 

Blue  “ 

0.137 

0.274 

Zinc. 

Blue  “ 

0.144 

0.288 

This  table  is  shown  graphically  on  Fig.  24. 


1 The  echelon  shows  that  the  separation  of  the  components  of  D2  to  be  about  two- 
thirds  of  that  of  Dv  The  value  here  found  belongs  to  Z)v  the  line  having  the  greater 
magnetic  shift. 


No.  2.] 


INTERFEROMETER  STUDY. 


41 


Fig.  21. 


Fig.  22. 


42 


JOHN  C.  SHEDD. 


[Vol.  IX. 


0 0.1  0.2  0.3 

MAGNETIC  SHIFT  A"  A'  IN  A.U.- 


Fig.  23. 


Fig.  24. 


No.  2.] 


INTER  PER  OME  TER  S TUB  Y 


43 


RATIO  OF  IONIC  CHARGE  TO  IONIC  MASS. 


(I 

for  the  case  of  circularly  polarized  light.  This  must  be  slightly 
modified  for  plane  polarized  light  as  the  period  of  single  vibration 
will  be  half  that  of  the  complete  vibration  of  the  circularly  rotating 
ion.  Equation  (io)  will  then  be  written 

X — )J  2TCV 
e/m  = -jj-  ' 

The  numerical  value  in  electromagnetic  units  may  now  be  calcu- 
lated from  the  values  of  \ — 1 ' and  H. 

For  the  lines  so  far  examined  the  values  are  as  follows  : The  lines 
may  be  grouped  according  to  the  value  of  e/m ; this  gives  the  fol- 
lowing interesting  table. 


Equation  (io)  gives  the  relation 

X — /'  AfiV 
e!m  — yi  yy 


Table  XI. 


Substance. 

Line. 

e[m 

Sodium. 

Yellow. 

22.45  X 10s 

Mercury. 

Vio  et. 

23.81 

Cadmium. 

Blue. 

22.41 

Zinc. 

Blue. 

23.46  “ 

Mercury. 

Green. 

18.59  “ 

Cadmium. 

Green. 

17.48 

Mercury. 

Yellow. 

14.35 

Cadmium. 

Red. 

11.93 

1 


Type  of  line. 
(Michelson.) 


Type  II. 


Type  III. 
Type  I. 


It  is  thus  seen  that  the  groupings  according  to  Michelson’s  chart 
(see  Fig.  3)  and  according  to  the  value  of  ejm  are  the  same. 

The  data  furnished  by  Preston1  are  too  meager  to  furnish  a just 
comparison,  but  it  is  to  be  said  that  his  classification  is  not  so  well 
defined  as  is  that  furnished  by  the  interferometer. 

Polarization. — The  method  of  observing  the  state  of  polarization 
has  been  outlined.  In  the  lines  examined  the  state  of  polarization 
1 Ph.il.  Mag.  (5),  45,  p.  330.  See  part  I. 


44 


JOHN  C.  SHEDD. 


[Vol.  IX. 


was  found  to  be  normal  with  the  exception  of  two  cases,  that  seem 
deserving  of  notice. 

I.  Violet  Line  of  Mercury. — In  the  readings  of  Table  VIII.  the  set- 
tings were  taken  successively  on  the  yellow,  green  and  violet  lines  for 
the  same  value  of  H,  by  shifting  the  slit  5 in  Fig.  io.  The  nicol 
was  so  set  as  to  cause  the  fringes  to  expand,  thus  giving  the  re- 
tarded component.  For  high  values  of  A the  behavior  of  the  fringes 
in  the  case  of  faint  lines  is  not  readily  observed,  and  it  was  not  until  the 
value  A = 5-14  mm.  was  reached  that  it  was  noticed  that  the  violet 
line  contracted,  while  the  yellow  and  green  lines  expanded.  This 
behavior  was  carefully  observed  for  the  balance  of  the  readings 
and  persisted  throughout.  This  peculiar  state  of  polarization  would 
indicate  that  the  component  of  the  violet  line  which  agrees  in  rota- 
tion with  the  magnetic  whirl  is  retarded  instead  of  accelerated,  as  in 
the  other  lines.  Hence  we  must  have  present  a species  of  diamag- 
netic ion  in  contrast  to  the  ordinary  or  “ magnetic  ” ion.  The  above 
observations  were  taken  with  the  greatest  of  care,  and  yet  the  con- 
clusion seemed  so  unusual  as  to  call  for  repeated  verification.  All 
attempts,  however,  to  secure  the  same  observations  proved  unavail- 
ing so  far  as  reversed  polarization  was  concerned. 

II.  In  the  case  of  the  cadmium  lines  the  following  observations 
were  made  : The  red , green  and  blue  lines  were  examined,  the  nicol 
being  set  so  as  to  cause  the  fringes  to  contract,  thus  giving  the  ac- 
celerated ray.  The  observations  were  begun  with  H — 2,900  C.  G. 
S.  units.  At  the  second  reading  with  H — 6,600  the  fringes  all  ex- 
panded, and  so  also  for  H=  7,900.  At  the  last  reading  with  H — 
8,800  all  the  fringes  again  contracted.  The  apparent  conclusion  to 
be  drawn  is  that  a reversal  of  the  state  of  polarization  takes  place 
during  the  progress  of  the  experiment.  It  is  more  likely,  however, 
that  in  this  case,  since  (as  already  stated)  the  glass  of  the  vacuum 
tube  acts  as  a retardation  plate,  some  change  of  stress  due  either  to 
variation  in  temperature  or  to  the  magnetic  field,  is  responsible  for 
the  observed  effect.  This  fact,  in  connection  with  the  difference 
in  wave-length  between  the  violet  and  the  green  and  yellow  lines  of 
mercury,  may,  perhaps,  also  explain  the  unusual  behavior  observed 
in  this  line.  Both  these  cases  illustrate  the  necessity  of  carefully 
watching  the  instrumental  factors  present. 


No.  2.] 


INTERFEROMETER  STUDY. 


45 


Summary. — The  study  of  the  magnetic  shift  and  the  values  of  elm 
lead  to  the  following  conclusions  : 

I.  A classification  of  lines  according  to  the  amount  of  magnetic 
shift  is  of  little  value. 

II.  A classification  of  lines  according  to  the  value  of  e\m  is 
significant. 

III.  Such  a classification  groups  the  lines  according  to  the  type 
of  line  produced,  as  given  by  the  analysis  of  visibility  curves. 

IV.  The  smaller  the  value  of  ejm  the  less  the  broadening  of  the 
component  lines  and  the  simpler  their  structure ; vice  vei'sa  the 
larger  the  value  of  ejm  the  more  the  broadening,  and  the  more 
complex  their  structure. 


Recapitulation. 

In  the  preceding  pages  the  history  of  the  subject  has  been  traced, 
from  the  first  experiments  of  Faraday  to  the  present  time.  The 
bearing  of  the  Maxwell-Lorentz  theoiy  upon  the  progress  of  Zee- 
man’s experiments  is  noted,  and  the  reacting  influence  of  experiment 
upon  theory  is  shown,  so  that  in  the  papers  of  Larmor,  Lorentz  and 
others  the  mathematical  exposition  of  the  subject  has  kept  pace  with 
the  experimental  observations.  In  the  experimental  development  of 
the  problem  two  well-marked  methods  have  been  presented  and  com- 
pared. It  has  been  found  that  the  spectro-photographic  method  is 
(i)  limited  in  range  by  reason  of  the  small  resolving  power  of  the 
ruled  grating  when  so  minute  changes  are  to  be  measured  ; and  (2) 
is  limited  in  accuracy  by  reason  of  the  wide  margin  of  error  in  the 
settings  of  the  micrometer,  especially  when  nebulous  lines  are  to  be 
measured. 

On  the  other  hand,  the  interferometer  method  has  been  seen  to 
possess  a resolving  power  greatly  in  excess  of  the  photographic 
method,  and  hence  is  applicable  to  low  values  of  H as  well  as  to  high 
values. 

The  results  accomplished  so  far  consist  in 

I.  By  the  spectro-photographic  method. 

(1)  A classification  of  lines  according  to  the  type  of  line  produced 
by  the  magnetic  field. 


46 


JOHN  C.  SHEDD. 


[Vol.  IX. 


(2)  The  measurement  of  the  distance  between  outside  compo- 
nents of  magnetic  triplets  and  a determination  of  the  value  of  the 
ratio  e\m. 

The  number  of  lines  so  far  examined  is  not  large,  nor  do  the 
values  of  the  magnetic  shift  obtained  by  different  observers  seem  to 
agree  very  well ; but  enough  has  been  done  to  suggest  a classifica- 
tion of  lines  parallel  to  that  obtained  by  Kayser  and  Runge. 

II.  With  the  interferometer  Professor  Michelson  has  presented, 

(1)  Three  well-marked  types  of  lines,  with  a possible  fourth  type. 

(2)  He  has  concluded  that  the  magnetic  shift  is  to  be  regarded 
as  approximately  independent  of  substance  or  color. 

The  chief  value  to  be  attached  to  his  work  is  contained  in  (1), 
and  it  would  seem  to  us  better  to  leave  (2)  unformulated  rather  than 
to  state  it  as  an  approximate  law. 

The  experiments  described  in  this  paper  have  sought  to  deter- 
mine, 

(1)  The  relation,  at  different  temperatures,  of  the  magnetic  shift 
to  strength  of  field. 

(2)  To  present  a method  of  measuring  the  magnetic  shift  that 
shall  be  as  free  as  possible  from  objections  involved  in  existing 
methods. 

The  method  adopted  is  similar  to  existing  interferometer  methods, 
but  is  believed  to  be  especially  adapted  to  the  present  problem. 

(3)  To  measure  the  magnetic  shift  and  determine  the  ratio  ejtn. 

The  values  obtained  for  magnetic  shift  show  that  any  classifica- 
tion based  upon  this  alone  is  of  relatively  small  value ; but  that  a 
classification  based  upon  the  value  of  e\m  is  significant. 

This  classification  is  seen  to  give  groups  of  lines  identical  with  the 
groups  presented  by  Michelson’ s three  types  of  lines. 

The  complexity  of  structure  is  also  seen  to  depend  upon  the 
value  of  this  ratio  e\m. 

The  preceding  experiments  were  carried  on  in  connection  with 
the  Fellowship  in  Physics  from  1897  to  1899  at  the  University  of 
Wisconsin,  under  the  direction  of  Professor  Benjamin  W.  Snow ; 
whom,  in  conclusion,  I desire  to  thank  for  his  helpful  and  continued 
encouragement  throughout  the  progress  of  this  investigation. 

Physical  Laboratory,  University  of  Wisconsin,  April,  1899. 


No.  2.] 


INTERFER OME  TER  STUD  Y 


47 


Appendix  I. 

Extract  from  Life  of  Faraday  by  Dr.  Bence  Jones,  Vol.  II.,  p.  44. 

“ 1862  was  the  last  year  of  experimental  research  ; Steinheil’s  ap- 
paratus for  producing  the  spectrum  of  different  substances  gave  a 
new  method,  by  which  the  action  of  magnetic  poles  upon  light 
could  be  tried.  In  January  he  made  himself  familiar  with  the  appa- 
ratus, and  then  he  tried  the  action  of  the  great  magnet  on  the  spec- 
trum of  NaCl,  BaCl,  StCl  and  Li  Cl, 

“ On  March  12  he  writes : 

“ ‘Apparatus  as  on  last  day  (January)  but  only  ten  pairs  of  voltaic 
battery  for  the  electromagnet. 

“ ‘ The  colorless  gas  flame  ascended  between  the  poles  of  the  mag- 
net, and  the  salts  of  sodium,  lithium,  etc.,  were  used  to  give  color. 
A Nicol’s  polarizer  was  placed  just  before  the  intense  magnetic  field, 
and  an  analyzer  at  the  other  extreme  of  the  apparatus.  Then  the 
electromagnet  was  made,  and  unmade,  but  not  the  slightest  trace  of 
effect  on  or  change  in  the  lines  of  the  spectrum  was  observed  in 
any  position  of  polarizer  or  analyzer. 

“ ‘ The  other  pierced  poles  were  adjusted  at  the  magnet,  the  colored 
flame  established  between  them,  and  only  that  ray  taken  up  by  the 
optic  apparatus  which  came  to  it  along  the  axis  of  the  poles,  i.  e., 
in  the  magnetic  axis  or  line  of  magnetic  force. 

“ ‘ Then  the  electromagnet  was  excited  and  rendered  neutral,  but 
not  the  slightest  effect  on  the  polarized  or  unpolarized  ray  was  ob- 
served.’ 

“This  was  the  last  experimental  research  that  Faraday  made.” 

II. 

Extract  from  M.  Ch.  Fievez  (Astronome  a l’Observatoire  Royale 
de  Bruxelles),  Bulletins  de  /’ Ac ade mi e Royale  de  Belgique , 3d  serie, 
tome  ix.,  p.  381  (1885). 

“ L’installation  spectroscopique  de  l’Observatoire,  disposant  d’un 
appareil  dispersif  de  tres  grande  puissance  et  d’un  electro-aimant 
Faraday,  construction  Ruhmkorff,  pouvant  etre  active  par  un  cou- 
rant  de  50  ampres  d’intensite,  a permis  d’aborder  ce  probleme. 


48 


JOHN  C.  SHEDD. 


[Vol.  IX. 


“ La  flamme  oxyhydrique  d’un  petit  chalumeau  etait  dirigee 
horizontalement  sur  un  charbon  sode  place  entre  les  armatures 
coniques  de  l’electro-aimant,  distantes  Tune  de  l’autre  de  io  milli- 
metres. Une  image  de  la  flamme  etait  projetee  sur  la  fente  du 
spectroscope  par  un  objectif  double.  La  quantite  d’oxygene  intro- 
duce dans  cette  flamme  permettait  de  regler  la  temperature  de  facon 
a donner  aux  raies  spectrales  et  D2  l’apparence  voulue. 

“ Dans  ces  conditions,  les  raies  sodiques  Dx  et  D2  etant  d’abord 
peu  larges  et  non  renversees  avant  le  passage  du  courant  d’aiman- 
tation,  deviennent  immediatement  plus  brillantes , plus  longues  et  plus 
larges  aussitot  que  l’electro-aimant  est  mis  on  activite. 

“ Si  les  raies  brillantes  Dx  et  D2  sont  deja  elargies,  1’ electro -aimant 
etant  inactif,  elles  deviennent  plus  larges  encore  et  se  renversent 
(c’est-a-dire  qu’  une  raie  noire  parait  au  milieu  de  la  raie  brillante 
elargie)  pendant  le  passage  du  courant  d’aimantation. 

“ Si  les  raies  sont  deja  elargies  et  renversees,  l’elargissement  de 
la  raie  brillante  et  de  la  raie  noire  devient  beaucoup  plus  considerable. 

“ Ces  phenomenes,  qui  disparaissent  instantanefnent  lors  de  V inter- 
ruption du  courant,  peuvent  etre  observes,  mais  avec  moins  d’in- 
tensite,  sur  la  raie  rouge  du  potassium,  du  lithium,  sur  la  raie  verte 
du  thallium,  etc.,  lorsqu’  une  minime  quantite  de  ces  metaux  ou 
d’un  de  leurs  sels  est  placee  sur  le  support  de  charbon. 

“ Enfin,  les  armatures  coniques  de  l’electro-aimant  etant  rem- 
placees  par  les  armatures  meplates,  de  maniere  que  toute  la 
longueur  de  la  flamme  sodique  seit  comprise  entre  ces  armatures, 
les  raies  Dx  et  D2,  prealablement  renversees  et  elargies,  presentent  un 
double  renversement  (c’est-a-dire  l’apparition  d’une  raie  brillante  au 
milieu  de  la  raie  noire  elargie),  lorsque  l’electro-aimant  est  en 
activite.” 


III. 

Extract  from  article  in  Astro.  Phys.  Jr.,  February,  1899,  by 
Perot  and  Fabiy. 

“ * * * when  the  light  is  complex  it  is  easy  to  obtain  a precise 
measure  of  the  ratio  of  the  wave-length  of  the  radiations  which 
constitute  it;  let  there  be  two  radiations  of  nearly  equal  wave- 
length / and  X — £.  The  distances  between  the  silvered  surfaces  is  in- 


No.  2.] 


INTERFEROMETER  STUDY. 


49 


creased  until  the  discordance  between  the  two  systems  of  rings  is 
complete.  Then  if  e is  the  distance  between  the  surfaces  (which  is 
given  with  sufficient  accuracy  by  the  micrometer)  we  have 

_i- A 

l 4 £ 

“ * * * This  method  is  also  readily  adapted  to  the  study  of  the 
change  of  wave-length  of  a given  line,  on  condition  that  the  radia- 
tion be  sufficiently  monochromatic ; in  such  a case  a comparison 
can  be  made  of  two  sources  emitting,  for  instance,  in  the  one  case 
the  altered  radiation  and  in  the  other  the  normal  radiation,  atten- 
tion being  directed  to  the  change  in  the  appearance  of  the  rings 
produced  by  the  two  sources  successively.” 

Note. — With  the  interferometer  the  ray  travels  the  distance  be- 
tween the  plates  twice  and  hence  J = 2e.  Therefore  the  equation 
for  the  interferometer  becomes 

e A A2 

— = -or  e = — . 


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